LFTGN 03
Lfd Landfill directive
www.environment-agency.gov.uk
Guidance on the management of landfill gas www.environment-agency.gov.uk
The Environment Agency is the leading public body protecting and improving the environment in England and Wales.
It’s our job to make sure that air, land and water are looked after by everyone in today’s society, so that tomorrow’s generations inherit a cleaner, healthier world.
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Published by: Statement of Use Environment Agency This guidance is one of a series of documents relating to the Rio House, Waterside Drive, Aztec West management of landfill gas. It is issued by the Environment Almondsbury, Bristol BS32 4UD Agency and the Scottish Environment Protection Agency (SEPA) Tel: 01454 624400 Fax: 01454 624409 as best practice guidance and will be used in the regulation of landfills. It is primarily targeted at regulatory officers and the © Environment Agency September 2004 waste industry. It will also be of interest to contractors, consult- All rights reserved. This document may be reproduced with ants and local authorities concerned with landfill gas emissions. prior permission of the Environment Agency. The document provides an update to Waste Management Paper This report is printed on Cyclus Print, a 100% recycled stock, 27. which is 100% post consumer waste and is totally chlorine free. Environment Agency and SEPA officers, servants or agents accept Water used is treated and in most cases returned to source in no liability whatsoever for any loss or damage arising better condition than removed. from the interpretation or use of the information, or reliance on Dissemination Status views contained herein. It does not constitute law, but officers Internal: Released to Regions may use it during their regulatory and enforcement activities. External: Public Domain Any exemption from any of the requirements of legislation is not Environment Agency’s Project Team: implied. The following were involved in the production of this guidance: Throughout this document, the term ‘regulator’ relates jointly to Chris Deed Head Office (Project Manager) the Environment Agency and SEPA. SEPA does not necessarily Mark Bourn North East Region support and is not bound by the terms of reference and recom- Peter Braithwaite Head Office mendations of other documentation mentioned in this guidance, Ged Duckworth Head Office and reserves the right to adopt and interpret legislative require- Jan Gronow Head Office ments and appropriate guidance as it sees fit. The term ‘Agency’ Mark Maleham Head Office should therefore be interpreted as appropriate. Jill Rooksby Midland Region Alan Rosevear Thames Region Richard Smith Head Office SEPA Project Team Rowland Douglas John McFeat Overview of the guidance
The EU Landfill Directive 1999/31/EC, which came into force on 16 July 1999, aims to improve standards of landfilling across Europe by setting specific requirements for the design and operation of landfills, and for the types of waste that can be accepted in landfills. All landfills are required to comply with the Directive’s requirements, although a transitional period is allowed for landfills existing at 16 July 2001. In England and Wales, the Directive is implemented through the Landfill Regulations (England and Wales) 2002, made under the Pollution Prevention Control Act 1999. In Scotland, the Directive is implemented through the Landfill (Scotland) Regulations 2003, as amended.
The role of the regulator is to condition the operation of an installation under the Pollution Prevention Control (PPC) Regulations 2000 by issuing a PPC permit. Once a PPC permit is issued, the regulator ensures that its conditions are met until such times as the regulator accepts its surrender. The regulator fulfils a similar role for landfill sites operating under waste management licensing.
This document, which updates Waste Management Paper 27 Landfill gas (DoE, 1991a), has been prepared to provide clear and concise guidance on the management of gas from landfill sites. The document sets out the legislative requirements of the Landfill Regulations, the PPC Regulations, the Waste Framework Directive and current good practice. This guidance will form the basis for setting conditions in PPC permits (including landfill permits) that provide for all appropriate measures to be taken against pollution, to limit emissions and impact on the environment, and when setting appropriate conditions in waste management licences. Future revisions of this guidance will further develop Best Available Techniques for landfill gas utilisation.
Readers of this guidance are expected to be familiar with the Landfill Directive requirements and the national regulatory frameworks. This includes the Defra guidance, IPPC: a practical guide (Defra, 2002a), which sets out how Government expects the PPC regime to operate in England and Wales. In Scotland the relevant PPC guidance is, The Pollution Prevention and Control (Scotland) Regulations 2000: a practical guide that is issued by the Scottish Executive Environment and Rural Affairs Department and SEPA.
This overarching guidance on the management of landfill gas is supported by a number of specific guidance documents. The series comprises:
● Guidance on landfill gas flaring ● Guidance for monitoring enclosed landfill gas flares ● Guidance for monitoring landfill gas engine emissions ● Guidance for monitoring trace components in landfill gas ● Guidance for monitoring landfill gas surface emissions (in England and Wales); ● Guidance on gas treatment technologies for landfill gas engines.
Scope This guidance sets out a structured approach to the management of all gases generated from landfilled waste. It covers the assessment of landfill gas impacts, the implementation of control methods and the monitoring required to demonstrate proper performance of the control measures.
Environment Agency Guidance on the management of landfill gas 1 The document consists of three parts:
● Part A sets out the regulatory framework under which landfill gas is to be managed. ● Part B sets out the legislative requirements for landfill gas management and the role of risk assessment. ● Part C provides technical information and details of current best practice on landfill gas management.
Where technical standards explicitly required by the Landfill Regulations (England and Wales) for landfill gas management and control are referred to in this document, they are highlighted in a box for clarity.
2 Environment Agency Guidance on the management of landfill gas Contents
PART A: Regulatory framework 1. Regulatory framework 7 1.1 Introduction 7 1.2 The Landfill Directive 7 1.3 The Landfill (England and Wales) Regulations 2002 7 1.4 The Landfill (Scotland) Regulations 2003 8 1.5 Waste management licensed landfill sites 8 1.6 Unlicensed (closed) landfill sites 8 1.7 Pollution Prevention and Control Regulations 2000 8 1.8 Environment Agency’s strategy for regulating landfill gas 9 1.9 Planning and development 10 1.10 Other regulations and guidance 11 PART B: Legislative requirements and risk assessment 2. Risk assessment 17 2.1 A risk-based strategy 17 2.2 Risk assessment framework 17 2.3 The conceptual model, hazard identification and risk screening 19 2.4 Tier 2 and Tier 3: simple and complex quantitative risk assessments 28 2.5 Accidents and their consequences 31 2.6 Monitoring and reviews 33 2.7 Global warming potential 34 3. Gas Management Plan 36 3.1 Definition 36 3.2 Objectives 36 3.3 Framework for the Gas Management Plan 36 4. Requirements for gas control 40 4.1 Introduction 40 4.2 Gas containment 40 4.3 Gas collection 42 4.4 Utilisation, flaring and treatment 42
Environment Agency Guidance on the management of landfill gas 3 5. Requirements for monitoring 44 5.1 Introduction 44 5.2 Monitoring at the site preparation phase 46 5.3 Monitoring during site operational and aftercare phases 46 5.4 Other guidance 47 PART C: Technical considerations 6. Landfill gas production and emissions 51 6.1 Composition of gas from biodegradable waste 51 6.2 Landfill gas characteristics 53 6.3 Gas production 56 6.4 Landfill gas emissions 63 7. Gas control measures 69 7.1 Design and construction quality assurance 69 7.2 Containment 70 7.3 Gas collection 72 7.4 Utilisation, flaring and treatment 80 8. Monitoring 85 8.1 Source monitoring 85 8.2 Emissions monitoring 86 8.3 Monitoring air quality 90 8.4 Meteorological monitoring 92 8.5 Monitoring procedures 92 8.6 Data analysis and reporting 94 Appendices Appendix A: Identified trace components in landfill gas 96 Appendix B: Compositional comparison of gas sources 100
Appendix C: Effects of CO2 on the flammable limits of methane 101 Appendix D: Average flare emissions data 102 Appendix E: Example emergency procedures 103 Appendix F: Characteristics of various gas sensors 105 Glossary 109 Acronyms 113 References 114 Index 118
4 Environment Agency Guidance on the management of landfill gas A
Guidance on the management of landfill gas
Part A: Regulatory framework
Environment Agency Guidance on the management of landfill gas 5 6 Environment Agency Guidance on the management of landfill gas 1
Regulatory framework
1.1 Introduction (amongst other things to reduce global warming) through a reduction in the landfilling of The management of landfill gas at permitted landfills biodegradable waste and requirements to introduce is covered by three pieces of European legislation: landfill gas control. ● Waste Framework Directive (75/442/EEC as This will, in part, be achieved by: amended) ● Landfill Directive (1999/31/EC) ● reducing the amount of biodegradable municipal ● Integrated Pollution Prevention and Control (IPPC) waste (BMW) disposed of to landfill; Directive (96/61/EC). ● banning the deposit of certain wastes in landfills; ● pretreating most wastes prior to disposal in For permitted landfills in England and Wales, these landfill. Directives are implemented by the Landfill (England and Wales) Regulations 2002 and the Pollution Implementation of the Landfill Directive will result in Prevention and Control (England and Wales) a reduction of the volume of gas generated from Regulations 2000, both of which were made under waste. Changes in the waste composition may also the Pollution Prevention and Control (PPC) Act 1999. result in significant changes both in the generation and constituent components of landfill gas. In Scotland, these Directives are implemented by the Landfill (Scotland) Regulations 2003 (as amended) The Landfill Directive sets out three classes of landfill: and the Pollution Prevention and Control (Scotland) ● landfill for inert waste Regulations 2000 (as amended), which were both ● landfill for non-hazardous waste made under the PPC Act. ● landfill for hazardous waste. The Waste Management Licensing Regulations 1994 The Landfill Directive provides minimum requirements (as amended) still apply to landfills, which are closed, for the design and operation of all classes of landfill, have ceased accepting waste and are unable to enter including landfill gas control. It defines landfill gas as into the PPC regime, but remain licensed. ‘all the gases generated from landfilled waste’. Based on this definition, landfill gas includes gases 1.2 The Landfill Directive generated by the biodegradation of waste and those The overall aim of the Landfill Directive as expressed arising from chemical reactions and the volatilisation in Article 1 is: of chemicals from the waste. by way of stringent operational and technical requirements on the waste and landfills, to provide for 1.3 The Landfill (England and Wales) measures, procedures and guidance to prevent or Regulations 2002 reduce as far as possible negative effects on the environment, in particular the pollution of surface The technical requirements of the Landfill Directive water, groundwater, soil and air, and on the global have been implemented in England and Wales via the environment, including the greenhouse effect, as well Landfill (England and Wales) Regulations 2002. The as any resulting risk to human health, from landfilling general requirements for all classes of landfills are set of waste, during the whole life-cycle of the landfill. out in Schedule 2 of the 2002 Regulations. Schedule 2 paragraph (4) of the Regulations requires the Item 16 of the recital to the Landfill Directive following gas control measures. intimates that measures should be taken to reduce the production of methane gas from landfills
Environment Agency Guidance on the management of landfill gas 7 1.6 Unlicensed (closed) landfill sites (1) appropriate measures must be taken in order to control the accumulation and migration of landfill gas; Closed landfills that do not have a waste management licence issued under Part II of the (2) landfill gas must be collected from all landfills Environmental Protection Act 1990 may fall within receiving biodegradable waste and the landfill gas must be treated and, to the extent possible, used; the definition of contaminated land contained in Part IIA Section 57 of the Environment Act 1995. Section (3) the collection, treatment and use of landfill gas under 78A(2) of the 1995 Act defines contaminated land for sub-paragraph (2) must be carried on in a manner, the purposes of Part IIA as: which minimises damage to or deterioration of the environment and risk to human health; and Any land which appears to the local authority (4) landfill gas which cannot be used to produce energy in whose area it is situated to be in such a must be flared. condition, by reason of substances in, on or under the land, that: The requirements for landfill gas control set out in the (i) significant harm is being caused or there is Landfill Regulations are, with the exception of minor the significant possibility of such harm being text changes, the same as the requirements in Annex caused; or 1 (4) of the Landfill Directive. Chapter 7 provides guidance on how the requirements for landfill gas (ii) pollution of controlled waters is being or control are to be met. likely to be caused. The statutory guidance uses the concept of a ‘pollutant linkage’, i.e. a linkage or pathway between 1.4 The Landfill (Scotland) Regulations 2003 a contaminant and a receptor. Landfill gas from old, The technical requirements of the Landfill Directive unlicensed sites may form part of a significant are implemented in Scotland via the Landfill pollutant linkage. (Scotland) Regulations 2003. The general The Environment Agency is not responsible for the requirements for all classes of landfills set out in the regulation of closed unlicensed landfill sites and, as Regulations re-iterate the requirements of the Landfill such, this guidance document represents good Directive. practice for landfills permitted under the PPC or the waste management licensing regime. However, local authorities that are responsible for unlicensed landfill 1.5 Waste management licensed landfill sites under Section 57 of the Environment Act 1995 sites may find this guidance a useful source of best Landfill sites that hold waste management licences practice. will continue to be regulated under the Waste Management Licensing Regulations 1994 (as amended) until such time as the regulator accepts 1.7 Pollution Prevention and Control surrender of the licence if the landfill: Regulations 2000 ● is deemed closed before implementation date of The Integrated Pollution Prevention and Control the Landfill Directive at 16 July 2001; or (IPPC) Directive has been implemented in England and Wales through the Pollution Prevention and ● has not been granted a PPC permit after the Control (England and Wales) Regulations 2000 (made submission and consideration of a site under the PPC Act 1999). In Scotland, it has been conditioning plan and where application for a PPC implemented through the Pollution Prevention and permit has been made or an appropriate closure Control (Scotland) Regulations 2000. notice has been served. The IPPC regime employs a permitting system to Sites that closed after 16 July 2001 will have to achieve an integrated approach to controlling the comply with the Landfill Directive and subsequent environmental impacts of certain industrial activities. regulations in relation to site closure and aftercare. An important feature of the IPPC Directive is the Much of the guidance presented in this document requirement on the regulator to ensure, through will therefore also apply to sites regulated under a appropriate permit conditions, that installations are waste management licence. operated in such a way that all the appropriate preventive measures are taken against pollution, in particular through application of the Best Available Techniques (BAT). BAT is defined in Regulation 3 of
8 Environment Agency Guidance on the management of landfill gas the PPC Regulations and matters that must be 1.8 Environment Agency’s strategy for considered when determining BAT are set out in regulating landfill gas Schedule 2 of the Regulations. The Environment Agency’s strategy for the future However, the condition making powers of the PPC regulation of landfill gas is focused on environmental Regulations are largely dis-applied by the Landfill outcomes and is based upon the concept of emission- (England and Wales) Regulations 2002 (Landfill based regulation. This strategy will improve the Regulations) in respect of landfilling activities. Instead, performance and regulation of landfill gas the relevant technical requirements of the Landfill management systems in three ways. Regulations, together with its condition making ● Operators will be required to detail their proposed powers, will cover the aspects of the construction, landfill gas control methods, monitoring, operation, monitoring, closure and surrender of procedures and actions through the development landfills. of a site-specific Gas Management Plan. This will Notwithstanding the situation with regard to the be reviewed annually and revised in the light of landfill, landfill gas utilisation plant may be regulated updates of the risk assessment and recent individually by the Agency in England and Wales monitoring data. under the PPC Regulations as a combustion activity ● Operators will be required to measure the burning fuel manufactured from or comprising a emissions from landfill gas flares, engines and waste other than waste oil or recovered oil. The landfill surfaces. These emissions will be assessed threshold for such control is plant with a thermal against emission standards. input of greater than 3 MW. Landfill gas utilisation ● Landfill sites, including the gas management plant may also be regulated by the Agency through a system, will be inspected regularly. This will be landfill permit, where it forms part of the installation. underpinned by the use of detailed site audits, Although BAT cannot be applied to the activity of including the use of Agency check monitoring. landfilling, the principles of BAT should be applied in These procedures will be introduced as landfill sites the landfill permit to directly associated activities and are permitted or re-permitted in accordance with the other listed non-landfill activities. PPC regulatory regime (or sooner if site-specific risk In England and Wales, there will also be determines that improvements should be completed circumstances where the landfill and the gas earlier). management system require separate permits while For closed sites where a waste management licence still being part of the same installation. A separate remains in force, the Agency will require the licence PPC permit may be required if the gas management holder to produce a landfill gas Emissions Review, system is operated by another party (albeit under which will be based on the development of a risk contract to the operator of the landfill). This is screening/conceptual model of gas management for because PPC permits can only be issued to ‘operators’ the site. Where this review identifies unacceptable of installations or mobile plant and the operator is site-specific risks from landfill gas, the licence holder defined as ‘someone who has control’ of its operation will be required to prepare an emissions improvement (or who will have control in the case of proposed programme that incorporates appropriate best plant) (Environment Agency, 2001a). This is practice from this guidance. This replacement interpreted to mean someone who has direct day-to- programme will be undertaken on a risk basis, for day control over the activities (i.e. landfill gas completion as soon as reasonably practicable, and as management). identified by the site-specific Emissions Review. The This guidance forms the basis for setting conditions in improvements identified in an Emissions Review must PPC permits (including landfill permits) which provide be completed at all Agency-regulated landfills by 16 all appropriate measures to be taken against July 2009. pollution, to limit emissions and the impact on the In the future, type approval for landfill gas engines environment and when setting appropriate conditions and flares may form an important part of this landfill in waste management licences. This guidance is best gas strategy. UK waste management companies are practice; it does not cover all aspects of BAT as set out keen to see the development of a type approval in Schedule 2 of the PPC Regulations. The Agency system, as they consider it would provide a safer and intends to review the guidance and to further develop more efficient method of achieving the appropriate best available techniques for landfill gas utilisation. standards. Under this system, specific landfill gas flare and engine models could be shown to be capable of
Environment Agency Guidance on the management of landfill gas 9 meeting the emission standards set by the Agency identified above may require an EIA, and advice from and could be demonstrated to do so reliably in the the relevant planning authority should be sought. field environment. The Agency supports a move Under the General Development Procedure Order towards this approach as it may lead to more cost- 1995 as amended (GDPO), the regulator is a effective monitoring of landfill gas combustion statutory consultee to the planning process on landfill equipment, in the knowledge that the Agency’s sites, including planning issues associated with landfill emissions standards were being met. gas. Where it is appropriate and legitimate, the regulator encourages co-ordinated applications for planning 1.9 Planning and development permission and PPC permits. This facilitates the 1.9.1 Development of new or current operational continual development of the conceptual model and landfill sites technical details of the proposal through the planning and permitting process within a consistent framework Land-use issues at new or current operational landfill (see Chapter 2 on risk assessment). sites are controlled by the local planning authority (LPA) in England and Wales and the planning The regulator envisages that an Environmental authority in Scotland. They are responsible for Statement prepared to the standards required for the establishing a planning policy framework which EIA Regulations will, for most non hazardous and meets the objectives of sustainable development hazardous landfills, constitute a Tier 2 risk assessment under the Town and Country Planning Act (1990) that will also form part of the PPC permit application. England and Wales, and the Town and Country However, it is likely that the risk assessment prepared Planning Act (Scotland) 1997. for the planning application will need to be extended and revised to reflect the detailed engineering design Planning consent is required for landfill developments and operation of the site, and to accommodate any before the regulator grants a PPC permit following a changes brought about by conditions within the new application for operation of the site. Guidance planning consent. on planning and waste management licensing for England is given in Planning Policy Guidance 10 Planning authorities are responsible for deciding an (PPG10) Planning and waste management (ODPM, application for planning permission. This decision 1999) and in Planning Policy Guidance 23 (PPG 23) may be to approve, to approve with conditions, or to Planning and pollution control (ODPM, 2002). refuse. In Scotland, advice is provided within National Planning Policy Guideline 10 (NPPG 10) Planning and 1.9.2 Development on or adjacent to landfill waste management (Scottish Office, 1997). Advice is sites in England and Wales also provided through Planning Advice Note 51 (PAN The Town and Country Planning (General 51) Planning and environmental protection (Scottish Development Procedure) Order 1995 (GDPO) (as Office, 1997) and Planning Advice Note 63 (PAN 63) amended) requires the planning authority to consult Waste management planning (Scottish Executive, with the Agency before granting planning permission 2002a). for development within 250 metres of land which is As a result, landfill gas and its management has to be being used for the deposit of waste (or has been at taken into consideration during the planning process. any time in the previous 30 years) or has been Most landfill developments, and in particular those notified to the planning authority for the purposes of involving inputs greater than 50,000 tonnes per year that provision. and/or occupying an area of 10 hectares or greater, The Agency has developed ‘standing advice’ to are likely to require an environmental impact enable LPAs to make decisions on planning assessment (EIA) in support of a planning application applications for hard development within 250 metres under Schedule 2 of the Town and Country of a ‘licensed’ or ‘permitted’ landfill without Environmental Impact Assessment Regulations consulting the Agency. The standing advice should be (England and Wales) 1999. In Scotland, these a material consideration in determining the planning requirements are contained in the Environmental application, as would advice received from the Impact Assessment (Scotland) Regulations 1999. Agency under Article 10(5) of the GDPO The EIA must include the potential impact of landfill (Environment Agency, 2003a). gas on the surrounding environment, including the For landfill sites that are no longer licensed or air quality and the visual impact of gas flares and permitted, the Agency holds information that it can utilisation plants. Sites smaller than the criteria
10 Environment Agency Guidance on the management of landfill gas provide to local authorities to aid decision-making on 1.10 Other regulations and guidance relevant planning applications. Local authority records This guidance has been compiled primarily to address for landfill sites that closed prior to the requirements the requirements of the Landfill Regulations 2002 and of waste management licensing regime are likely to the PPC Regulations 2000. A list of other legislation, be more complete than those held by the Agency. which may have an impact on the control or management of landfill gas, is given in Table 1.1. Table 1.1 Current legislation relevant to the control and management of landfill gas
Title
EU Directives 75/442/EEC Waste Framework Directive and amendments 96/61/EC Integrated Pollution Prevention and Control 97/11/EC Environmental Impact Assessment 91/689/EEC Hazardous Wastes 99/31/EC Landfilling of Waste 96/62/EC on air quality assessment and management 1999/30/EC relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead 2000/69/EC relating to limit values for benzene and carbon monoxide in ambient air
Acts Environmental Protection Act 1990 Town and Country Planning (England and Wales) Act 1990 Town and Country Planning (Scotland) Act, 1997 Clean Air Act 1993 Environment Act 1995 Pollution Prevention and Control Act 1999
Statutory Landfill (England and Wales) Regulations 2002, SI 2002 No. 1559 Instruments Landfill (Scotland) Regulations 2003 SI 2003 No. 235 Pollution Prevention and Control (England and Wales) Regulations 2000, SI 2000 No. 1973 Pollution Prevention and Control (Scotland) Regulations 2000, SI 2000 No. 323 Air Quality (England) Regulations 2000, SI 2000, No. 928 Air Quality (Wales) Regulations 2000, WSI 2000, No. 1940 Air Quality (Scotland) Regulations 2000, SSI 2000, No. 97 Air Quality (Scotland) Amendment Regulations 2002, SSI 2002, No. 297 Air Quality Limit Values Regulations 2001, SI 2001 No. 2315 Air Quality Limit Values (Wales) Regulations 2002, WSI 2002 No. 3183 Air Quality Limit Values (Scotland) Regulations 2001, SSI 2001, No. 224 Waste Management Licensing Regulations 1994, SI 1056 as amended Control of Substances Hazardous to Health Regulations 2002, SI 2002, No. 2677 Contaminated Land (England) Regulations 2000, SI 2000, No. 227s Contaminated Land (Wales) Regulations 2001, WSI 2001, No. 2197 Town and Country Planning General Development Order 1988, SI 1998, No. 1813 Town and Country Planning (General Permitted Development) Order 1995, SI 1995, No. 418 Town and Country Planning (General Development Procedure) Order 1995, SI 1995, No. 419 Town and Country Planning (General Development Procedure (Scotland) Order 1992 (as amended) Town and Country (Environmental Impact Assessment) (England and Wales) Regulations 1999, SI 1999, No. 293 Environmental Impact Assessment (Scotland) Regulations 1999, SSI 1999, No. 1 Planning (Control of Major-Accident Hazards) (Scotland) Regulations 2000, SSI 2000, No. 179
Environment Agency Guidance on the management of landfill gas 11 Table 1.1 Current legislation relevant to the control and management of landfill gas (continued)
Title
Guidance Planning Policy Guidance Note 10 Planning and waste management, 1999 Planning Policy Guidance Note 22 Renewable energy, 1993 Planning Policy Guidance Note 23 Planning and pollution control Planning Guidance (Wales) Planning policy, 1999 (under review) Technical Advice Note (Wales) 8 Renewable energy, 1996 Technical Advice Note (Wales) 21 Waste, 2001 National Planning Policy Guideline 6 Renewable energy, 2000 (applies in Scotland) National Planning Policy Guideline 10 Planning and waste management, 1996 (applies in Scotland) Planning Advice Note 51 Planning and environmental protection, 1997 (applies in Scotland) Planning Advice Note 58 Environmental impact assessment, 1999 (applies in Scotland) Planning Advice Note 63 Waste management planning, 2002 (applies in Scotland) Planning Advice Note 45 Renewable energy technologies (revised 2002) (applies in Scotland)
1.10.1 Air quality and odour control The regulator takes into account the Air Quality Strategy and its objectives when setting licence or Air quality permit conditions. In addition, Council Directive Occupational air quality limits that apply to the 1999/30/EC set limit values for the concentration of workplace are established for a range of compounds for sulphur dioxide, nitrogen dioxide and oxides of that are present in landfill gas. These are listed in the nitrogen, particulate matter and lead in ambient air Health and Safety Executive (HSE) Guidance Note and dates for their attainment. Where the impact of EH40 (HSE, 2002), which is reviewed annually. an installation is likely to lead to a breach of these limit values, the operator may be required to go Local authorities are responsible under the Air Quality beyond BAT to comply with the limit values. Strategy for England, Scotland, Wales and Northern Ireland to ensure that the public is not exposed to an air quality that poses a significant risk to human Odour health or quality of life. The strategy addresses the following air pollutants. The Waste Framework Directive requires that waste is recovered or disposed of without causing nuisance ● benzene through noise or odour. In addition, the Landfill ● 1,3-butadiene Regulations require the following. ● carbon monoxide ● lead Measures shall be taken to minimise nuisances ● oxides of nitrogen arising from the landfill in relation to: ● ozone ● ● particulates (PM10) emissions of dust and odours; ● sulphur dioxide. ● wind-blown material; ● noise and traffic; The Strategy lays down objectives to be met by 2005 ● birds, vermin and insects; for each pollutant. Several of the listed pollutants may ● the formation of aerosols; be associated with landfill gas and emissions from the ● fires. gas management system. Local authorities have a critical role in delivering these objectives through Odour can present potential health issues (even if action plans as part of the Local Air Quality only in perception) and it is important to remember Management regime. This involves the review and that the regulator has powers and duties to address assessment of air quality in their area against the human health issues in addition to pollution and objectives. Where the objectives are unlikely to be amenity issues. met, local authorities must develop action plans that set out how they can be obtained.
12 Environment Agency Guidance on the management of landfill gas Guidance has been produced on odour control for landfills in England and Wales (Environment Agency, 2002a). Odour is discussed in more detail in Chapter 6. Odour and its impacts, if deemed a statutory nuisance, can be regulated by the environmental health departments of local authorities under Sections 79–80 of the Environmental Protection Act 1990. This allows local authorities to serve abatement notices on ‘premises’ where a statutory nuisance exists or is likely to occur or recur. Abatement notices can require abatement of the nuisance or the prohibition of its occurrence and actions to remedy the situation. COSHH The Agency is not responsible for the regulation of health and safety at landfills. The Control of Substances Hazardous to Health Regulations (COSHH) 1988, 1994, 1999 and 2002 place a legal obligation on landfill operators to demonstrate that the workforce engaged in landfill operations is not exposed to a significant health risk. This includes those directly employed by the landfill operator and those employed as contractors to undertake secondary tasks such as engineering or monitoring. The Regulations require employers to comply with the occupational exposure limits established by the HSE and to undertake studies to assess the risk posed to workers where the level of exposure is unknown.
1.10.2 Clean Air Act 1993 Local authorities may require that a planning application is submitted for the installation of landfill gas flares or gas utilisation plant. They may also wish for an application for chimney height approval to be made under the Clean Air Act 1993. This would require landfill gas flare/combustion operators to submit evidence that the height of the flare or exhaust stack is sufficient to prevent emissions being prejudicial to human health or likely to cause a nuisance.
Environment Agency Guidance on the management of landfill gas 13 14 Environment Agency Guidance on the management of landfill gas B
Guidance on the management of landfill gas
Part B: Legislative requirements and risk assessment
Environment Agency Guidance on the management of landfill gas 15 16 Environment Agency Guidance on the management of landfill gas 2
Risk assessment
2.1 A risk-based strategy framework for environmental risk assessment and management is described in DETR et al. (2000). This The Agency’s strategy for the future regulation of landfill consists of a tiered approach where the level of effort gas is based on environmental outcomes. This places put into assessing each risk is proportionate to its great emphasis on emissions monitoring and magnitude and its complexity. A conceptual approach compliance assessment. The strategy augments, but to the tiered risk assessment is shown in Figure 2.1. does not replace, the existing philosophy of best practice regulation of landfill gas infrastructure, which This process emphasises the: retains a key role. ● importance of developing a robust conceptual site At a fundamental level, this strategy requires an model at the risk screening stage, based on a understanding and quantification of landfill gas through source–pathway–receptor approach that is risk assessment and the development of a conceptual continually reviewed and updated as new model of the site. The conceptual model and proposed information is collected; level of risk assessment should be the subject of early ● need to screen and prioritise all actual and pre-application discussions with the regulator. potential risks before quantification; ● need to match effort and resources in evaluating The risk assessment approach involves: potential risks to the magnitude of environmental ● the assessment of potential impacts on local damage that could result from each hazard; environment, health and amenity; ● need for an appropriate level of measures to ● the development of a Gas Management Plan. manage the risks; ● iterative nature of the process, with annual reviews The Gas Management Plan includes: being an integral part. ● management options, procedures and collection Operators of existing landfill sites transferring to the efficiency determination; PPC regulatory regime as part of transitional ● emissions monitoring and assessment from various arrangements will be required to undertake a risk parts of the landfill gas infrastructure. assessment to support their PPC application. Operators The feedback of monitoring and assessment should apply an appropriate tier of risk assessment in information enables the validation/improvement of accordance with the guidance given below. For non- both the conceptual model and the Gas Management hazardous and hazardous waste landfill sites, this is Plan. This provides opportunities for improvements likely to require the application of Tier 2 or Tier 3 risk based on environmental outcomes. assessments. The risk assessment must address the key issues raised by the Landfill Regulations (see Box 2.1). For a new site, 2.2 Risk assessment framework these issues are dealt with by the planning authority at The regulator requires the use of a structured approach the planning application stage. They are likely to be the to the assessment of the risks posed by a landfill to subject of the EIA submitted with the application. The human health, the environment and local amenity. This regulator is a statutory consultee at the planning stage is a pre-requisite for the permitting of all landfills under and is likely to have a view on all of these issues. PPC and a fundamental part of preparing a Gas For an existing site, the pathways and receptors identified Management Plan. The ongoing assessment of risk for in the box should be on the list of those considered in operational sites will be a requirement for the the risk assessment. This should demonstrate that there maintenance of a PPC permit. will be no unacceptable emissions from the site in both Risk assessment should be a transparent and practical the short and the long term. process that aids decision-making. The recommended
Environment Agency Guidance on the management of landfill gas 17 Box 2.1 Risk assessment issues identified by the Landfill Regulations
The location of a landfill must take into consideration requirements relating to: ● the distances from the boundary of the site to residential and recreational areas, waterways, water bodies and other agricultural or urban sites; ● the existence of groundwater, coastal water or nature protection zones in the area; ● the geological/hydrogeological conditions in the area; ● the risk of flooding, subsidence, landslides or avalanches on the site; ● the protection of the natural or cultural heritage in the area. The Landfill Regulations also require that: ● landfill gas must be collected from all landfills receiving biodegradable waste and the landfill gas must be treated and, to the extent possible, used. ● the collection, treatment and use of landfill gas is required and must be achieved in a manner, which minimises damage to or deterioration of the environment and risk to human health.
The assessment of risk from landfill gas and gaseous with landfill gas. The Gas Management Plan should emissions must be developed in conjunction with the be developed from the risk assessment. Continuous risk assessment for aqueous emissions. The guidance review of site investigations and monitoring data below focuses on the assessment of gaseous risks. produced as part of the Gas Management Plan will indicate whether: The risk assessment is likely to develop in four stages: ● the data validate the conceptual model; Stage 1: Hazard identification and risk screening – ● there is a need to modify/update both the the initial development of the conceptual model and conceptual model and the Gas Management Plan. provides the basis for pre-application discussions for planning applications or for existing sites seeking a Stage 4: Completion – a thorough review of the PPC permit. conceptual model and monitoring data will be undertaken to determine whether the site meets the Stage 2: Simple quantitative risk assessment – surrender test and to confirm that it no longer poses submitted in support of the planning application and any pollution risk. forming part of the EIA for the site or in support of a PPC permit application. The risk assessment should follow a tiered approach as shown in Figure 2.1 and outlined in the following Stage 3: Complex quantitative risk assessment – sections, which provide guidance on the level of risk submitted in support of a PPC permit application for assessment necessary to provide sufficient confidence sites where a Stage 2 approach is not sufficient due to to allow decision-making. either the significance of risks posed by landfill gas at the site or the complexity of the issues associated
18 Environment Agency Guidance on the management of landfill gas Figure 2.1 Conceptual approach to risk assessment for landfill gas (adapted from DETR et al., 2000)
Conceptual model Stages
Initial site planning Tiered risk assessment START POINT Tier 1 Hazard identification and risk screening
Development of Gas Management Plan Tier 2 Simple Risk identification quantitative risk and prioritisation assessment Permitting
Tier 3 Complex risk assessment Operational and permit modification
Risk management assessment
Closure and aftercare
Decisions, measures and actions
2.3 The conceptual model, hazard ● the operational management and gas control identification and risk screening practices to be employed; ● an identification of the pathways to receptors Conceptual model including release (emission) points/areas for This is the initial development of the conceptual site landfill gas and combustion products. model and involves the development of an The conceptual model developed for the purposes of understanding of the landfill site in its surroundings. the landfill gas risk assessment must not be produced The conceptual site model should identify the nature in isolation from that required for the hydrogeological of the site and its planned development. With respect risk assessment. There should be one conceptual to landfill gas, it should include the following model for the landfill, which covers all environmental information: media. The development of a conceptual site model is an iterative process; the model must be reviewed ● the nature of the waste and the source term and updated as new information becomes available including: or as the understanding of the system is improved. – an initial indication of the likelihood of gas This process will form part of the required periodic production; review. – whether gas extraction is required; – an estimation of the likely volumes of gas The conceptual model should consist of site plans, generated; outline designs and sketches; these may be – whether utilisation is proposed. underpinned by spreadsheets and standard templates ● the environmental setting in which the site is listing locations and similar details. The initial located including identification of all receptors conceptual model will provide a useful platform for (including the atmosphere); discussions between the site operator and the ● an initial selection of appropriate environmental regulator as part of the pre-application process. By benchmarks, e.g. Environmental Assessment Levels the time an application for a permit is submitted, the (EALs) and odour thresholds; conceptual model should be based on detailed site ● a design of the containment, collection and plans and designs contained in the permit application treatments systems; and the Gas Management Plan.
Environment Agency Guidance on the management of landfill gas 19 Hazard identification and risk screening a landfill gas hazard. The emphasis in the risk assessment should, therefore, be placed on the Waste The hazard identification and risk screening stage Acceptance Procedures and particularly the waste should have the following overall objectives: characterisation and compliance monitoring measures ● development of an understanding of the proposed introduced to ensure that only inert waste is or existing landfill in its environmental setting (the deposited at the site. If these measures can be shown conceptual model described above), including to be robust, then the landfill gas source should be identification of all possible sources of risk, the demonstrably negligible. Provisions for the pathways and the potential receptors; monitoring of gas within the waste body will ● consideration of the sensitivity of receptors and normally be required at inert waste landfills (see initial selection of the appropriate environmental Chapters 5 and 8). benchmark, e.g. EALs for each receptor or groups For existing sites, previous deposits of non-inert waste of receptors; introduce a greater level of complexity and may ● consideration of the potential impacts on each require a more detailed level of assessment as receptor. This may be achieved by using a simple outlined below. quantitative or qualitative process that systematically examines each Biodegradable waste landfills source–pathway–receptor linkage and determines An initial approximation of the landfill gas generation the potential impact. For each receptor, this for sites that have taken biodegradable waste can be analysis should prioritise the risks and the produced by simply assuming that each tonne of requirements to be evaluated for further risk biodegradable waste will produce 10 m3 of methane assessment. per year. The equation shown in Box 2.2 can then be The following sections deal with each of these used to calculate the approximate gas flow that outputs. would be generated. This equation produces an overestimate of gas flow at peak production and gas 2.3.1 The source of risk flow from historic waste deposits. More sophisticated The source term for all landfills needs to be models of gas generation (including GasSim; see considered and assessed during risk screening and Environment Agency, 2002b) can also be used at this hazard identification. This applies whether or not the stage. landfill site accepts inert, non-hazardous or hazardous wastes, and whether the waste is biodegradable or Box 2.2 Calculating the approximate gas flow inorganic. However, the approach for assessing the source term may be different depending on the type Q = M x 10 x T /8760 of landfill (see below). Where: 3 If landfill gas generation cannot be shown to be Q = methane flow in m /hour negligible, then this will trigger the best practice M = annual quantity of biodegradable waste in tonnes T = time in years requirements for active extraction, utilisation, flaring and the provision of barriers to minimise gas movement (see Chapter 4). These measures must be A predicted methane flow (Q) that exceeds a set out in the conceptual model and the Gas 3 Management Plan, and their impact assessed. simplistic benchmark value of 50–100 m /hour provides an initial indication that flaring or utilisation A number of factors influence gas generation and a will be required. These measures will then need to be variety of models can be employed for predicting built into both the conceptual model and the Gas rates of gas production (see Section 2.4). For the Management Plan. As the technology for flaring and purposes of risk screening, an approach is outlined utilising landfill gas develops, the regulator will revise below that will give a first estimate of the likely this simple benchmark. requirements for different types of landfills and for existing waste deposits. One necessary output from If the proposed mix of wastes or depth of waste is this initial screening is to determine whether landfill predicted to generate landfill gas but only at levels gas is a significant concern. that make collection inefficient, this may be an unacceptable landfill proposal. The proposed mix of Inert waste landfills wastes must have a viable landfill gas risk For inert landfills, the landfill gas risk assessment will management scheme, i.e. a low level of not normally have to progress beyond the risk biodegradable waste input would not be permitted screening stage. New inert landfills ought not to pose without a coherent gas management strategy.
20 Environment Agency Guidance on the management of landfill gas Where it is clear from the risk screening process that application must be fully evaluated and supported by landfill gas will be generated in more than negligible further risk assessment work. For example, the quantities, this will trigger the introduction of specification of the side wall lining needed to control measures that implement best practice requirements sub-surface migration will depend on the specific for the management of landfill gas. These pathways identified. These will also govern the requirements are discussed in more detail in Chapter migration monitoring requirements in terms of 4. These measures must be included in both the location and frequency. developing conceptual model and the Gas The timing for beginning active gas extraction is a Management Plan, and their impact evaluated. key risk management provision at all landfill sites/cells Best practice requirements for landfill gas are: where this is required. Where the risk assessment demonstrates that flaring is necessary, the first flare ● Containment – barriers to prevent sub-surface (or utilisation plant if appropriate) should, as an migration and minimise surface emissions of indicative standard, be operational within six months landfill gas. of waste being accepted into the site or cell. This ● Collection – an active gas extraction system to period may be extended if the waste input rate is low achieve the maximum practicable collection or specific site conditions make it likely that there will efficiency. The annual collection efficiency for be insufficient gas to sustain flaring. However, it is methane should be compared against a value of best practice to design cells in such a manner that 85 per cent. The operator or regulator may use waste emplacement and final capping can be this simple assessment to trigger further achieved quickly, thus facilitating early gas collection. investigation. This collection efficiency should be achieved in that part of the landfill where gas The following indicative benchmarks (Environment collection must be taking place (i.e. the capped Agency, 2000a) can be applied during the risk areas of the site). Box 2.3 shows how to estimate screening process to indicate when landfill gas collection efficiency. utilisation is likely to be required. The presumption ● Utilisation, flaring and treatment – a system of is that landfill gas must be utilised and, if any one combustion processes (or other treatment of the indicative benchmarks listed in Table 2.1 is processes) meeting the emission limits for that met, then landfill gas utilisation should be process. Treatment of the gas stream pre- or post- considered. These criteria are considered in more combustion will be a site-specific issue based on detail in Chapter 4. Where comparison against the precise composition of the gas stream. these benchmarks indicates that utilisation is required, then measures will be required in order to The measures to be applied to meet these demonstrate best practice requirements (see requirements will be determined by site operators in above). consultation with the regulator. However, their
Box 2.3 Estimating collection efficiency
Collection efficiency (E) = Mass of gas collected/Mass of gas produced E = A / (A + B + C) Where: A = mass of methane sent to utilisation and treatment B = mass of methane emitted through cap C = mass of methane lost by lateral migration through liner C is related to B by the relative permeability of the cap and the liner; this can be estimated from first principles and knowledge of design of engineered containment or, alternatively, through GasSim using information on the construction of the cap and liner. If K is the relative amount of gas emitted through the cap compared with that through the liner, K = (Permeability of cap) x (Surface area of cap)/(Permeability of liner) x (Surface area of liner above saturated zone). Substitution in the equation above gives: E = A / (A + B(1+K)) Parameters A and B are reported by the site operator. Parameter K can be calculated from information provided within the risk assessment.
Environment Agency Guidance on the management of landfill gas 21 Table 2.1 Indicative benchmarks for landfill gas utilisation
Parameter Value
Total quantity of emplaced waste ≥ 200,000 tonnes Gas flow rate ≥ 600 m3/hour Depth of waste ≥ 4 metres Waste composition ≥ 25% weight/weight (w/w) organic wastes
Inorganic waste landfills within the installation: Inorganic landfills that have not accepted ● dates of landfill commencement and any cessation biodegradable waste will produce a landfill gas of of deposits (both temporary and final); different composition to that produced from ● a summary of waste types and quantities in broad biodegradable landfills. This gas may not contain categories, e.g. inert (based on the Landfill methane and carbon dioxide in the bulk proportions Directive definition), construction and demolition, normally associated with gas from biodegradable household, industrial, commercial and special; landfills. It is, however, possible that gaseous ● an estimate of the biodegradable fraction of the emissions (landfill gas) (e.g. hydrogen) will still be waste deposited; produced from the inorganic wastes deposited. This ● a summary of landfill gas monitoring records will also apply to the separate cells used for the within and external to the waste body; disposal of hazardous wastes. ● all trace gas analyses and a summary of the analytical results; Rigorous waste acceptance procedures including ● gas control measures currently applied and gas waste characterisation and compliance monitoring generation rates (including pumping trials); will be necessary at such sites to ensure that ● odour complaints or recorded instances of odour biodegradable waste is not accepted. episodes. The method of determining this source term will be This information will allow an initial assessment of the site-specific and dependent upon the waste types existing waste deposits and their contribution to gas accepted by the site/cell. Unless it can be generation. demonstrated that the source and risks associated with the landfill gas are negligible, then extraction 2.3.2 Receptors and utilisation/flaring of the landfill gas will be A number of potential receptors need to be required. For certain landfills (e.g. in-house landfills considered with respect to landfill gas and the that are well characterised and monitored), there may conceptual model should identify site-specific be sufficient confidence at the risk screening stage examples of the generic categories. These are: that the risk is negligible, but a more detailed risk assessment is likely to be required. ● domestic dwellings (human occupation in bands: closer than 50 metres, between 50 and 250 Where gas collection and combustion are required, metres and 250 to 500 metres); the risk management measures introduced must be ● hospitals; capable of dealing with the uncertainty associated ● schools and colleges; with the source term. Monitoring of gas generation ● offices, industrial units and commercial premises; and composition will form a vital part of the iterative ● sensitive habitats and environmental areas e.g. risk assessment process in order to characterise this sites of special scientific interest (SSSIs); uncertainty. ● public footpaths or bridleways; Existing waste deposits ● major highways and minor roads; ● open spaces; Where PPC applications cover previously landfilled ● parks; areas, the historic waste inputs are unlikely to ● allotments; conform to the strict requirements for inert, non- ● farmland (e.g. crop damage); hazardous or hazardous landfills. Assessment of the ● air quality management zones. source term should be based on records of the wastes accepted and the results of landfill gas monitoring. Where the risks are likely to be the same (e.g. a particular street or small group of houses), it may be During the risk screening process, the following useful to group receptors together. outputs will be required for waste already deposited
22 Environment Agency Guidance on the management of landfill gas Each receptor or group of receptors should be Table 2.2 presents a selection of EALs that can be identified on the site plan. The information should be used at the risk screening stage for biodegradable set out on a standard template such as contained in non-hazardous landfills. The Agency has also Horizontal Guidance Note H1 (Environment Agency, identified a number of priority compounds that 2002c), where additional information on this subject should be considered with specific reference to is provided. This will create a transparent record for landfill gas. Estimated emissions at the site of each of the site-specific receptors that can then be receptors identified in the conceptual model should linked to the predicted impact, the risk management be compared with the EALs. measures and the monitoring requirements. At hazardous and inorganic landfills, a wider range of 2.3.3 The initial selection of environmental substances need to be considered depending upon benchmarks the proposed waste types. When selecting appropriate EALs for use as assessment levels at In order to determine the sensitivity of the existing sites, the trace gas composition and environment within the vicinity of a landfill, risk flare/engine emission data/limits should be screening should identify the most appropriate considered. Horizontal Guidance Note H1 environmental benchmarks to be applied. These may (Environment Agency, 2002c) provides useful include air quality standards, EALs and odour information in formulating these benchmarks. thresholds. Most of the environmental benchmarks available for These benchmarks should be used in the risk use in the risk assessment for releases to air are screening of each hazard (e.g. air quality, odour) currently based on occupational exposure data or associated with the specific emissions. A comparison odour threshold data for human receptors. There are of the concentrations in the environment resulting a number of short-term and long-term benchmarks from the emissions against these environmental available that are taken from EU air quality limit benchmarks will allow their significance to be assessed values and national objectives (e.g. for benzene, and a decision to be made on whether the landfill’s carbon monoxide, nitrogen dioxide, particulates impact is acceptable or whether further risk (PM10) and sulphur dioxide. The most appropriate assessment work is required. benchmark, e.g. for nitrogen oxides (NOx) and EALs for both the short-term (1-hour reference period) sulphur oxides (SOx), should be selected on the basis and long-term are given in Horizontal Guidance Note of the appropriate averaging period for the identified H1 (Environment Agency, 2002c). Releases (emissions) receptors. should be expressed according to any relevant For the sub-surface migration of landfill gas, an standard conditions and the statistical basis from appropriate environmental benchmark for methane which they are derived should be indicated. The same and carbon dioxide is 1 per cent and 1.5 per cent measurement period should be used for comparison volume/volume (v/v) above background, respectively. against environmental benchmarks. The consideration of site-specific background levels is Table 2.2 Example environmental benchmarks essential in the determination of absolute risk and for air quality* should not be ignored. Section 2.3.6 explains how these benchmarks should EAL (µg/m3) Emission be used in the receptor assessment process. Long-term Short-term 2.3.4 Pathway Combustion gases NO 2 Receptors may be exposed to landfill gas emissions Nitrogen dioxide (NO )40# 200# 2 through: Sulphur dioxide (SO2)50267 ● Carbon monoxide 350 10,000# direct release to atmosphere; ● Hydrogen chloride 20 800 sub-surface migration through the ground or Raw landfill gas along service ducts and/or pipelines, etc; ● indirect release to atmosphere, e.g. from sub- Hydrogen Sulphide 140 150 surface landfill gas migration, or dissolution from Benzene 16.25 208 leachate and condensate; Chloroethane 27,000 338,000 ● direct release of combustion products to 2-butoxyl ethanol 1,230 – atmosphere, e.g. enclosed flares and engines. Chloroethene (vinyl chloride) 159 1851 1,1-dichlorethane 8,230 165,000 Any landfill is likely to have a variety of potential release points and fugitive emissions related to landfill * Adapted from Environment Agency, 2002c. gas. The conceptual model must be based on the # See Table D1 (Environment Agency, 2002c).
Environment Agency Guidance on the management of landfill gas 23 reality of the situation for each landfill and the allowance for wind directions and topography) assessment of pathways must start from the should be considered and compared with suitable assumption that the appropriate best practice environmental benchmarks. This will indicate whether requirements are in place. Release points/areas will landfill gas poses a risk. include: The trace components of landfill gas pose an odour ● freshly deposited wastes; and toxicity risk, while the bulk gases pose a risk due ● the surface (cap) of the landfill (including areas of to explosion and asphyxiation (although carbon permanent and temporary capping, the dioxide is also toxic and should be considered in the intermediate batters to the landfill and the assessment of toxicity). Explosion and asphyxiation working area); risk is generally related to sub-surface migration and ● the interface of the landfill with the surrounding accumulations in enclosed spaces. This is more geology and engineering features; difficult to quantify and, for the risk screening stage, ● leaks from the gas and leachate collection systems the impact assessment should be based on: (pipework, valves, wells); ● the presence of potential pathways and site- ● gas and leachate treatment plant; specific receptors; ● degassing of leachate and condensate during ● a qualitative assessment of the severity of the collection and/or treatment; consequences. ● flare stacks; ● exhaust emissions from utilisation plant; Risk screening using simple air dispersion modelling ● intermittent emissions during excavations, well may employ the methodology reflected in Horizontal drilling, leachate pumping or other engineering Guidance Note H1 (Environment Agency, 2002c) to works. estimate the potential impact of the emissions. The following outlines the requirements at the risk The relative importance of each of these will vary on screening stage. a site-specific basis. All the potential site-specific release points/areas should be identified and listed in 2.3.6 Dispersion of emitted gas a standard format. A site plan should identify the Emission rate potential release points/areas. To quantify the effect of emissions, it is necessary to The releases identified through the risk assessment know the emission rate, e.g. grams per second (g/s). process will require management and monitoring, The landfill gas emission rate should be based on the and the standard template provides this link. For estimated generation rate from future deposits, i.e. it instance, a potential leak from a well head will require should reflect the period of maximum gas generation routine management inspection and monitoring. at the site. Where the well head is close to a receptor (e.g. a footpath), the frequency of monitoring may be For existing sites, determination of emission rates greater than where the well head is more remote. An should also consider monitoring data. The gas flow odour complaint from the receptor should feedback estimates should be used, together with an into further monitoring and a re-evaluation of risk assumption of gas stream composition, to produce an management measures. emission rate in grams per second. Where uncertainties exist in estimated values, it is usual to 2.3.5 Receptor prioritisation and impact assume the ‘worst case’ and to state any assumptions assessment made. The prioritisation process will be a qualitative For the purposes of risk screening, emissions should assessment based on consideration of the estimated be quantified for flares and engines. For existing sites, impact (roughly quantified as described below), the one option is to use monitoring data for the existing sensitivity of the receptor (qualitative) and the combustion plant. An alternative is to extrapolate likelihood of exposure (qualitative). data from a similar site and plant to the maximum The potential hazards that exist from landfill gas are: predicted gas flow. To ensure a conservative assessment, emissions from flares and engines for new ● toxicity (acute and chronic) sites should be based on typical emission levels ● ecotoxicity obtained from literature – although these should be ● fire and explosion rationalised by reference to the manufacturer’s ● asphyxiation published performance data. Unless there is auditable ● odour. evidence available of compliance, emission levels For the purposes of risk screening, potential emissions based simply on assumed compliance with Agency and simple air dispersion (including a general best practice emission limits should not be used for
24 Environment Agency Guidance on the management of landfill gas the risk assessment. Models such as GasSim The appropriate value for GLC can be obtained from (Environment Agency, 2002b) can be used to derive a Horizontal Guidance Note H1 (Environment Agency, release rate for risk screening. For GasSim, the value 2002c) and from tables such as Tables 2.3 and 2.4, used for the release rate should be the predicted which are specific for landfill gas engines/enclosed maximum 95th percentile emission rate over the life flares. The calculations in these tables assume typical of landfill site. exit velocities and emission temperatures for a range of stack heights, with off-site maxima dependent In the absence of site-specific design data, the default upon the distance from the stack to the landfill assessment of fugitive emissions should consider a boundary. These are derived from mathematical flux of 85 per cent of the theoretical generation for dispersion models and presented as maximum comparison with short-term environmental average GLC for unit mass emission rates for different benchmarks (e.g. short-term EALs) and 30 per cent stack heights. The predicted concentrations of short- for comparison with long-term environmental term and long-term releases are based on the use of benchmarks (e.g. long-term EALs; see Table 2.2). Both dispersion factors assuming ‘worst case’ situations, i.e. short-term and long-term effects should be highest concentration and impact. For long-term considered in the risk screening process. The releases, the GLCs are presented as maximum annual predicted impact should be calculated, as far as averages and, for short-term releases, as maximum possible, on the same basis as the corresponding hourly averages. environmental benchmark, e.g. over the same averaging period or percentile exceedence. All assumptions made and data sources used during Box 2.4 Estimating predicted concentrations in air the risk screening process should be recorded. The simple calculation method shown in Box 2.4 can PCair = GLC x RR be used for screening purposes, but it does not take Where: into account all the parameters that may influence 3 PCair = predicted concentration (µg/m ) dispersion of substances to air. This equation will generally provide a more conservative estimate of RR = release rate of substance in g/s ground level concentration (GLC) than more complex GLC = maximum average ground level concentration models. Fit-for-purpose models (including GasSim for for unit mass release rate (µg/m3/g/s), based on long-term impacts) may be used instead of the annual average for long-term releases and equation to provide estimates of the predicted hourly average for short-term releases. concentration for risk screening.
Table 2.3 Estimation of maximum off-site hourly GLCs for a unit mass emission rate (µg/m3 per g/s emitted)
Distance to site boundary (m) Gas engine height (m) Flare stack height (m)
5678 6810
<50 255 185 135 100 285 140 75 50 240 185 135 100 215 140 75 100 140 125 115 100 110 90 75 150 95 85 80 75 70 60 55 200 70 65 60 55 50 45 40 250 55 50 50 45 40 35 35 300 45 40 40 40 30 30 25 350 35 35 35 35 25 25 25 400 30 30 30 30 20 20 20 450 30 25 25 25 20 20 15 500 25 25 25 25 15 15 15
Environment Agency Guidance on the management of landfill gas 25 Table 2.4 Estimation of maximum off-site annual mean GLCs for a unit mass emission rate (µg/m3 per g/s emitted)
Distance to site boundary (m) Gas engine height (m) Flare stack height (m)
5678 6810
<50 13 11 9.5 8 7 5.5 4 50 13 11 9.5 8 7 5.5 4 100 13 11 9.5 8 7 5.5 4 150 9.5 9 8 7.5 5.5 5 4 200 7 6.5 6.5 6 4.5 4 3.5 250 5.5 5 5 5 3.5 3 3 300 444432.52.5 350 3.5 3.5 3.5 3 2.5 2 2 400 3 3 3 2.5 2 2 2 450 2.5 2.5 2.5 2.5 1.5 1.5 1.5 500 22221.51.51.5
Those substances that are emitted in such small assessing short-term effects, the short-term quantities that they will have an insignificant impact background should be taken to be equal to twice the on the receiving environment should be screened out long-term background. The total predicted at this stage (see Box 2.5). A summary table of their environmental concentration (PEC) of that substance predicted concentration (PC) should be produced is calculated by adding together the background and their significance assessed. This should be carried concentration and the contribution from the out using the method described below. engine/flare as identified in the equation in Box 2.6. The short-term and long-term PC of substances emitted should be compared with the relevant short- Box 2.6 Calculating the total PEC term and long-term environmental benchmarks for emissions to air. The same statistical basis for mass PEC = PC + Background concentration concentration as the environmental benchmarks must air air air be used to ensure a meaningful comparison. ● Long-term benchmarks – Modelling of long-term effects may be appropriate if the long-term PEC is Box 2.5 Is an emission insignificant above 70 per cent of the relevant environmental benchmark or, in locations where there is an Air An emission is insignificant where the PC is less than 1 Quality Management Plan, for the identified per cent of the environmental benchmark (long-term). substance. An emission is insignificant where the PC is less than 10 ● Short-term benchmarks – Modelling of short- per cent of the environmental benchmark (short-term). term effects may be appropriate if the short-term PC is more than 20 per cent of the difference Where the predicted concentration is judged between the short-term background concentration insignificant by these criteria, then the risk can be and the relevant short-term environmental regarded as negligible and need not be considered benchmark. further. Where the predicted concentration is judged In addition, nitrogen oxide and nitrogen dioxide are as potentially significant, it will be necessary to use commonly measured as NOx but the benchmarks are the following guidelines to decide whether detailed expressed as the individual constituents. In time, modelling is required. emissions of NO oxidise to form NO2 and the Information should be collected on the ambient following guidelines should be followed when concentrations for the appropriate substance. When assessing emissions from landfill gas engines and flares.
26 Environment Agency Guidance on the management of landfill gas ● For short-term impacts, convert all measured or 2.3.7 Recommendation for further risk estimated nitrogen oxide emissions to NO2 and assessment assume 50 per cent of this value when making Two further tiers of risk assessment can be conducted, comparisons with the short-term NO2 i.e. simple and complex. These correspond to Tiers 2 environmental benchmark. and 3 (see Figure 2.1) and are described in more ● For long-term impacts, convert all measured or detail in Section 2.4. The appropriate level of estimated nitrogen oxide emissions to NO2 and assessment will always be site-specific, but the use this value when making comparisons with the decision will be based on the confidence in the ability long-term environmental benchmark. of the risk assessment to address the risks and Gas migration uncertainties. This must be sufficient to allow the regulator to make a decision on the issue of the For those risks that cannot be quantified through air permit. dispersion (i.e. sub-surface migration), a qualitative assessment is required. Standard templates should be Risk screening (Tier 1) should be sufficient to deal completed for the comparison of predicted impact with most of the risks from inert sites. For non- against the environmental benchmark. hazardous and hazardous sites, a more detailed quantification of the impact of some emissions will The template should list the possible risks to each normally be required. Not all risks will, however, receptor and one of three possibilities should be require the same level of consideration and the recorded: prioritisation exercise during the risk screening should ● that the risk is negligible; make this clear. ● that the risk will be further quantified in a more The conceptual model should be discussed with the detailed assessment; regulator at the pre-application stage and the ● that a reference made to the risk management appropriate level of risk assessment agreed. However, measures that will be included in the application. risk assessment is an iterative process. Although a The template should list the risks in a prioritised simple assessment may be agreed, it will be necessary order, with the most significant risk for that receptor for the operator to apply a more complex risk listed first. The information can be presented assessment if there is a lack of confidence that the according to the example of the standard template risks have been addressed appropriately. shown in the Horizontal Guidance Note H1 More detailed modelling can be required to address (Environment Agency, 2002c) and should include, for the following factors: each substance, information on: ● the presence of sensitive receptors; ● its properties; ● the magnitude of error in initial estimates of ● release point; predicted concentration in relation to the ● PC for long-term and short-term emissions; potential; ● short-term and long-term environmental ● the predicted concentration compared to the benchmarks; environmental benchmarks; ● PC as a percentage of the relevant benchmark; ● background air quality. ● identification of significant emissions. Where receptors have been identified in the For sub-surface migration, a qualitative or semi- conceptual model (e.g. footpaths, domestic quantitative assessment should be carried out of the dwellings, schools and hospitals, etc.), then further risk of exceeding 1 per cent of the environmental assessment of the risks (including modelling work) benchmarks for specific pathways. will normally be required at the permit application For existing sites, the performance of the risk stage for all the non-negligible emissions identified management measures should form part of the through the risk screening process. consideration of whether a risk needs further Where the uncertainties are large (e.g. emissions from quantification. Any monitoring (e.g. external a new hazardous landfill), there will be a greater need boreholes) and incidents (e.g. odour complaints) for a complex risk assessment. The higher the should be used when assessing the potential impact predicted concentration in relation to the and the effectiveness of the current and proposed environmental benchmark, the more likely it is that a corrective (risk management) measures. detailed consideration of risk will be required.
Environment Agency Guidance on the management of landfill gas 27 2.4 Tier 2 and Tier 3: simple and complex models can encompass specific characteristics of the quantitative risk assessments landfill site and surrounding environment, e.g. air dispersion models that can account for particular Simple risk assessments types of terrain. Specialist and expert assistance is Simple risk assessments should be carried out for likely to be required for complex risk assessment. landfills when the previous risk screening is There are many uncertainties associated with the risk insufficient to make an informed decision on the risks assessment of landfill gas and the emphasis must be posed by the site. They should consist of quantitative on the use of best practice standards to minimise the calculations, typically solved deterministically using uncertainty risk. The objectives of Tier 2 and 3 risk conservative input parameters, assumptions and assessments are: methods. ● further quantification of the hazard, where the Simple risk assessments can be used when the source of emissions, the properties and potential source, pathway and receptor terms can be concentrations of the parameters of concern are defined with sufficient certainty such that they can be defined (e.g. hydrogen sulphide emitted from the confidently represented by conservative inputs, site surface, odour threshold and EAL); models and assumptions. Simple risk assessments will ● consideration of the timescales of the landfill gas generally be applicable in less sensitive locations generation and completion criteria with respect to where risk screening and prioritisation have not landfill gas; identified any receptors that would be particularly ● re-evaluation of the appropriate environmental susceptible to the consequences of landfill gas. A benchmark; complex risk assessment should be carried out when ● exposure assessment to: there is uncertainty about the source, pathway and – refine the understanding of receptors and the receptor terms, and a robust decision cannot be characteristics of the exposed populations made using conservative inputs, methods and – model emissions and pathways to the assumptions. population A Tier 2 risk assessment is a simple quantitative – estimate the concentration or dose to which process that examines the source–pathway–receptor the population may be exposed (e.g. links using a mixture of site-specific data and consideration of local topography and built projections of site performance based on either: environment); ● risk evaluation – an assessment of the significance ● generic sources of information (e.g. the rate of gas of the risk and its acceptability. This should generation and the expected composition of the provide a robust platform for decisions in the landfill gas); or management of risk. ● the range of values over which the parameters may vary are specified based on probabilistic 2.4.1 Gas generation and composition models, generic experience or generic field data. A Tier 2 or 3 quantitative risk assessment should Complex risk assessments rigorously examine the likely composition and volumes of the landfill gas produced. Complex risk assessments should be carried out when the site setting is sufficiently sensitive to warrant For existing sites, monitoring data should be used to detailed assessment and a high level of confidence is consider the bulk and trace composition of the gas. necessary to ensure compliance with legislation. They Further sampling should be undertaken where such should be carried out in a quantitative manner using information does not exist or is limited. The extent of stochastic (i.e. probabilistic) techniques leading to this additional sampling and analysis will be site- analytical or mathematical solutions. specific, but a complex assessment generally requires more extensive work. A complex risk assessment is effectively tailored to the characteristics of the specific site. The use of generic Parameters that should be considered under gas input data is therefore replaced, where appropriate, production include: by specific information derived from site monitoring, ● waste types site investigations and operational experience at the ● waste quantities site. The types of risk assessment tools likely to be ● rate of infill applied in complex risk assessment include fault and ● leachate and water management event tree analysis, and detailed dispersion models ● site geometry will be used to estimate the behaviour of released ● the rate of gas production components in the environment. Sophisticated ● period of gas production.
28 Environment Agency Guidance on the management of landfill gas A number of models can be used to predict gas Toxicological assessments generation (see Section 2.4.4), but their limitations The receptors can be subdivided into those that are should always be recognised when using their results. at risk through acute (short-term, event or fault- Prediction of trace gas composition is particularly driven) exposure and those that are at risk through difficult and, here, the use of probabilistic models can chronic (long-term) exposure. be useful. Alternative methods of hazard assessment should be Monitoring must be designed to test the assumptions used: regarding gas generation. Validation of models via site investigation and monitoring results should form ● where no appropriate air quality or environmental part of the annual review process. assessment thresholds are available for use as environmental benchmarks; 2.4.2 Completion ● pathways other than direct inhalation are being Agency guidance on landfill completion (Environment considered; Agency, 2004a) considers residual gas generation and ● multiple pathways are being examined. criteria for PPC permit surrender. Completion will be These methods generally provide a more complex achieved when passive fluxes no longer pose a risk to means of risk assessment than direct comparison with human health, the local environment or amenity in threshold and guideline data. They can make use of the short or long-term. human and animal toxicity data to produce a With respect to emissions that may lead to global ‘tolerable daily intake’ for the purpose of assessment warming, the overall principle is to maximise and hazard indices for assessing exposure by multiple methane oxidation to carbon dioxide over the entire pathways. Expert advice should be sought on the use life of the landfill. This will require long-term and application of appropriate toxicity data and collection and treatment. Assumptions should be models for this purpose. made about the efficiency of existing technologies Ecotoxicity such as low calorific burners and methane oxidation. These assumptions should be updated as part of the Root zone displacement of oxygen by landfill gas annual review; as technology advances, the ability to during lateral migration is the most likely cause of collect and treat at lower levels will improve and local ecotoxicity (Parry and Bell, 1983). Vegetation assumptions can be revised. stress is likely if the concentrations of methane and carbon dioxide in the soil exceed approximately 25.5 An estimate of the period of time predicted for active per cent and/or the oxygen concentration falls below gas extraction and treatment is required as 10 per cent. completion cannot be achieved until after such activities are no longer necessary. Risk assessment can facilitate the prediction of likely impacts of landfill gas releases upon local flora and 2.4.3 Revision of environmental benchmarks fauna. This may be particularly important where: selection ● agricultural land is adjacent to landfill sites The risk screening stage is intended to give a first ● residences with gardens lie alongside landfills indication of the level of risk posed by the site. Where ● landfills are near areas of special scientific or it is clear that the risks are low, sufficient confidence ecological interest, e.g. for protected habitats and may have been provided to enable decisions to be species. made at this stage. Where more complex assessments are required, a review of the source term and 2.4.4 Models receptors may indicate that more and/or different Simple and complex quantitative risk assessments environmental benchmarks need to be considered. make considerable use of computer models to predict The World Health Organization (WHO) has issued air gas generation and the dispersion of emissions. All quality guidelines for several compounds that are models are simplified representations of reality and found in landfill gas. These guidelines have no their output must always be considered in the light of statutory status and have been developed by the the assumptions, uncertainties and limitations of the WHO to provide a basis for protecting human health process. from the adverse effects of air pollution. The All input data should be referenced and their use guidelines provide background information designed justified. For example, gas generation rates derived to assist governments in making risk management from a kinetic model will contain assumptions based decisions, particularly when setting standards. on the type and mixture of waste to be accepted at
Environment Agency Guidance on the management of landfill gas 29 the site. These assumptions should be recorded on a tailored to an individual landfill site. This information standard template and form an important part of the is used by the model to estimate the generation of annual review process. Where the assumptions prove landfill gas at that site for any period up to 150 years to be inappropriate, this should trigger a re- in the future. The model also makes allowances for evaluation of the risk management measures. the diverse rates of degradation of differing waste components. Landfill gas flux to the environment (for Risk screening should deal with issues such as bulk and trace components) is then calculated, taking prevailing wind direction, while quantitative into account gas collection, flaring, energy recovery assessments should look in much more detail at and biological methane oxidation. Finally, the model dispersion issues. In particular, the risk assessment also includes an assessment of the health and must consider the meteorological conditions that environmental risks from a number of other bulk and usually result in low levels of dispersion and relate this trace landfill gas components, including a range of to the topography of the site (e.g. gas rolling down volatile organic compounds (VOCs). Using this model into valleys). in a simple risk assessment and/or as part of complex For environmental emissions, the analysis is risk assessment facilitates the quantitative evaluation undertaken with the support of computer-based required in the risk assessment process. models. These are designed to estimate the size and The use of quantitative risk assessment is an nature of the source (e.g. gas production models) important aid to judgement and decision-making, but and to simulate the distribution and transport of is not a mechanism in itself for making decisions. The pollutants from the source to receptors (e.g. air models employed will only be as reliable as the dispersion models). The Agency’s GasSim model quality of input data and will always contain an (Environment Agency, 2002b) can be used for this inherent level of uncertainty. The outputs, purpose. Other models can be used to examine assumptions and input parameters should be critically particular pathways, e.g. ADMS 3 (Cambridge re-examined if experience suggests that the results of Environmental Research Consultants, 2002) and the risk assessment differ from what is expected. The AERMOD are sophisticated air dispersion models. re-evaluation of assumptions will play an important There are circumstances where the use of dedicated part in the annual review process. and sophisticated models is necessary in order to 2.4.5 Impact assessment provide a reliable analysis of risk (see Section 2.4.5). These are likely to be situations where there are The emissions of substances from the proposed sensitive receptors (e.g. domestic dwellings) in close landfill development should be evaluated against the proximity to the landfill or point source emissions existing quality of the ambient air to assess potential (e.g. less than 500 metres). harm. This should be done by comparison of the long-term and short-term PEC of each substance It may be necessary to evaluate more than one released to air with its corresponding long-term and operating scenario and a variety of operational short-term environmental benchmark. The conditions in order to ensure that the impacts information should be presented according to the resulting from the worst case situation are assessed. example format given in Horizontal Guidance H1 GasSim (Environment Agency, 2002c) and recorded on a standard template. Such information includes: GasSim (Environment Agency, 2002b) was developed to assist risk management decisions about landfill gas ● PC; as part of the planning, regulation and operational ● background concentration (with location and aspects of a landfill site. GasSim can be used to help: measurement basis); ● corresponding short-term and long-term ● assess the risks from current or planned landfill gas environmental benchmarks, e.g. EAL or emissions; environmental quality standard (EQS); ● provide a framework that will contribute to the ● PEC; assessment and valuation of the inventory of ● comparison of PEC to the environmental burdens associated with the landfilling of wastes; benchmarks (EAL or EQS). ● the regulator and other relevant organisations compare the relative risks associated with different Sub-surface migration should be quantified on the landfill gas management options. basis of predicted leakage through the proposed barriers in a scenario where active extraction is not The user can define the mix, composition, occurring and a cap is in place. Predicted levels biodegradability, moisture content of the waste and should be compared to the environmental other parameters, thus allowing the model to be benchmarks derived at the risk screening stage.
30 Environment Agency Guidance on the management of landfill gas Any releases where the environmental benchmark value, there is a link between the process of setting (e.g. EAL or EQS) is already being breached, or where permit conditions under PPC and Local Air Quality the contribution from the installation will result in the Action Plans. environmental benchmark being breached, should be In addition, where a landfill is responsible for, or identified. contributes significantly to, a breach of a national air In addition, a number of relevant air quality standards quality objective, the subsequent emission standard that form part of the Government’s Air Quality may need to be more stringent than the Agency’s Strategy must be considered against the outputs of generic emission standards. However, where the the risk assessment. Many of these standards are relative contribution to the national air quality updated regularly and may vary in the different parts objective (where these are more demanding than EU of the UK. Regulations currently contain air quality limit values) is relatively minor, the landfill would be objectives for nine pollutant species (nitrogen dioxide, required to meet the generic emission standards for PM10 particulate matter, sulphur dioxide, ozone, lead, landfill gas engines and flares, but not go beyond carbon monoxide, benzene, 1,3 butadiene and them. polyaromatic hydrocarbons (PAHs)). These statutory Where the air quality strategy has adopted a objectives are based on standards in the EU Air European limit value as an objective but has set an Quality Daughter Directives and on the earlier compliance date than given in the EU directive, recommendations of the Expert Panel on Air Quality the need for measures which go beyond the generic Standards (EPAQS) and the WHO. standards to meet the limit value would be assessed Some of the pollutants have several objectives with in relation to the compliance date given in the lower ambient concentrations to be achieved in appropriate directive. different parts of the country over time; the dates for the objectives range from 2003 to 2010. The Government is required to achieve the objectives for 2.5 Accidents and their consequences ozone and PAHs, but the remaining objectives are for The risk assessment should examine a number of local authorities ‘to work towards achieving’ using a accident and failure scenarios in order to quantify the mechanism called Local Air Quality Management. In impact of given events. The scenarios to be this process, ambient levels of pollutants are assessed considered should be agreed with the regulator. The against the objectives; if compliance is unlikely, an Air more serious the impact, the greater the effort Quality Management Area is declared and an Air necessary to ensure – through risk management Quality Action Plan is constructed to achieve the measures – that the event will not occur. objectives. This process is repeated in cycles over time. The Agency takes account of the Air Quality The reliability of landfill gas control systems and site Strategy to ensure that the sites and installations it engineering should be assessed in the risk assessment regulates do not contribute significantly to any process. Where this is performed, the environmental exceedence of an air quality objective. analysis should be accompanied by fault tree and consequence analysis. This should cover the baseline In the subsequent determination of appropriate system/plant performance and all potential causes of emission standards for landfill gas engines and flares, failure. The main hazards that could lead to it is important to take into account the relative accidental emissions should also be identified. contribution of their combustion products to ambient local air quality (Environment Agency, 2000b). The EU The final output from a consideration of accidents Air Quality Framework and Daughter Directives place and their consequences should be one risk assessment a legal obligation on the UK to achieve the specified report for the landfill. air quality limit values for individual pollutants by the The general categories of accident for a landfill that required dates. In addition, the PPC Regulations would affect landfill gas control are: require that, where an environmental quality standard as set out in EU legislation (such as those in the air ● loss of containment, e.g. leakage, liner failure, quality directives) requires a stricter emission standard spillage; than would be achieved by applying BAT, the Agency ● loss of collection and/or treatment capability, e.g. should incorporate those stricter limits. When setting failure of pipework, well, equipment/control permit conditions, however, the Agency will take into system, etc.; consideration the extent to which controls upon ● explosions and fires, e.g. deep-seated landfill fire. other sources should reduce pollution levels to below the limit value. Therefore, in areas exceeding a limit
Environment Agency Guidance on the management of landfill gas 31 Examples of accident and failures scenarios that ● description of potential accident risk; should be considered include: ● substances emitted as a consequence of the accident; ● deep-seated fire and its effect on landfill gas ● maximum concentration of the accidental containment and emissions; emission; ● failure of leachate extraction systems and the ● medium to which emission is released; effect of this on landfill gas management; ● PC of the accidental emission. ● explosion of a flare; ● failure of gas treatment plant; For each release, a comparison of the PC of the ● extraction system failure. accidental emission to the short-term environmental benchmark (e.g. EAL or EQS) should For existing sites, the actual occurrence of incidents be made. Tables 2.5 to 2.8 can be used to assess can be used to inform the consideration of the likelihood, severity and risk. probability. Other sites managed by the operator can also be considered in this assessment, as For each of the incidents identified, a likelihood management practices are often standard across an category can be assigned as set out in Table 2.5. operator’s landfills. Using Table 2.6, an estimate of the severity of the The maximum concentration should be estimated for likely consequences can be made and the incident each emitted substance in the event of an accident. can be assigned an appropriate category. The resulting predicted concentration of the For each release, the overall risk can be evaluated by substance as dispersed into the ambient air should be multiplying the severity of consequence by its calculated using the methods described in Section likelihood (see Table 2.7). 2.3 and the information presented on a standard template. This information should include: For each incident, identify which of the score categories shown in Table 2.8 it falls into.
Table 2.5 Likelihood categories
Category Range 1 Extremely unlikely Incident occurs less than once in a million years 2Very unlikely Incident occurs between once per million and once every 10,000 years 3 Unlikely Incident occurs between once per 10,000 years and once every 100 years 4 Somewhat unlikely Incident occurs between once per hundred years and once every 10 years 5 Fairly probable Incident occurs between once per 10 years and once per year 6 Probable Incident occurs at least once per year Source: Environment Agency, 2002c
Table 2.6 Severity categories
Category Definition 1 Minor Nuisance on site only (no off-site effects) No outside complaint 2 Noticeable Noticeable nuisance off-site e.g. discernible odours Minor breach of permitted emission limits, but no environmental harm One or two complaints from the public 3 Significant Severe and sustained nuisance, e.g. strong offensive odours Major breach of permitted emissions limits with possibility of prosecution Numerous public complaints 4 Severe Hospital treatment required Public warning and off-site emergency plan invoked 5 Major Evacuation of local populace Temporary disabling and hospitalisation Serious toxic effect on beneficial or protected species Widespread but not persistent damage to land 6 Catastrophic Major airborne release with serious off-site effects Site shutdown Source: Environment Agency, 2002c
32 Environment Agency Guidance on the management of landfill gas Table 2.7 Severity likelihood matrixs
Severity of consequence Likelihood Minor Noticeable Significant Severe Major Catastrophic
Extremely unlikely 123456 Very unlikely 246810 12 Unlikely 369121518 Somewhat unlikely 481216 20 24 Fairly probable 51015202530 Probable 61218243036
Table 2.8 Risk evaluation ● provide an early warning of any departures from Magnitude of risk Score design conditions; ● Insignificant 6 or less give an early warning of adverse environmental impacts; Acceptable 8 to 12 ● provide an early warning of any breach of Unacceptable 15 or more emission standards; ● supply information to enable decisions on the The scores for releases to air and water can be added. management of the site to be taken; The cumulative risk score for the proposed landfill ● provide information to support an application for should be calculated and the results presented permit surrender. according to the template provided in Horizontal Information from monitoring programmes should be Guidance Note H1 (Environment Agency, 2002c). integrated into the conceptual model to: Negligible risks will score ‘insignificant’ in risk ● test assumptions in the conceptual model; evaluation. Any risks that score as ‘unacceptable’ ● evaluate reductions in the performance of should be eliminated or revised. Detailed modelling extraction systems and their effects on collection of the consequences of accidental releases should be efficiencies; undertaken. ● confirm that risk assessment and management The monitoring requirements of the Gas options are meeting their desired aims. Management Plan should be developed by The design of the monitoring plan for each landfill considering incidents as outlined above. Contingency should be based on the conceptual model and planning should also be linked to this risk assessment specific regulatory requirements. Identified pathways stage. and release points, both potential and actual, must be monitored. Monitoring is also required to evaluate any assumptions made in the risk assessment. 2.6 Monitoring and reviews The review process will largely be based on the results The main objectives of environmental monitoring are of monitoring and should aim to ensure that the to: input parameters, emissions and assumptions in the ● define a baseline against which to compare actual conceptual model are all realistic. For example, it is or predicted impacts; not possible to predict the trace gas composition of ● allow compliance with permit conditions to be the landfill gas accurately (particularly with changing assessed; waste types and landfill classifications). Monitoring ● confirm that engineering measures are controlling data on gas generation rates and the trace landfill gas (as they are designed to); composition of landfill gas are, therefore, an essential ● produce information on the processes occurring part of the continued development of the conceptual within the landfill; model, which is used in reviewing the risk assessment.
Environment Agency Guidance on the management of landfill gas 33 The risk assessment should direct the monitoring relative GWP. This parameter is a function of the strategy for: radiative properties of the gas and its atmospheric half-life, and may be defined as the time integrated ● validation of critical assumptions in the risk change (usually 100 years) in the radiative properties assessment, e.g. trace gas composition and gas of the atmosphere due to instantaneous release of generation rates; 1 kg of gas relative to that from 1 kg of CO2. The ● monitoring of identified release points, e.g. well GWPs for a variety of compounds found in landfill gas heads, pipework, engines, flares and surface flux; are given in Table 2.9. ● external monitoring, e.g. migration control boreholes and ambient air monitoring at The contribution of landfill gas from the landfill to receptors. global warming should be estimated, as part of the risk assessment process, using GWPs together with Monitoring locations, frequencies and determinands quantitative concentration and flux data. The should be justified in terms of the risk assessment and benchmark for global warming should be based on the identification of release points and pathways, achieving an annual 85 per cent efficiency for the receptors and critical assumptions. More information collection and treatment of methane emissions. on monitoring is given in Chapters 5 and 8. The PPC permit requires a periodic review of the performance of the landfill. Standard templates are important for the review process as they provide the link between the assumptions made and the real situation with the risk management provisions in place. Each potential release point should have been identified through the risk assessment process and the periodic review must consider the implications of the real data for each release. Consideration of odour complaints and/or incidents (such as fires or down times for flares and engines) is another vital part of the review. Odour problems indicate that landfill gas is not being collected and treated adequately. They should be recorded against the receptors at which the odour is detectable and must trigger an evaluation of the risk management measures.
2.7 Global warming potential Landfill gas, which contains principally methane and carbon dioxide, is a greenhouse gas and therefore contributes to global warming. The UK has made specific obligations under the Kyoto Protocol to reduce its emissions of greenhouse gases: the landfill industry’s contribution to these aims will be significant as landfills accounted for an estimated 27 per cent of the UK’s methane emissions in 2000 (Defra, 2002b). The global warming potential (GWP) of methane is approximately 21 times that of carbon dioxide. Therefore, efficient collection and combustion of landfill gas is required in order to protect the global atmosphere and environment. The potential contribution of individual chemicals to climate change can be quantified by reference to their
34 Environment Agency Guidance on the management of landfill gas Table 2.9 Ozone depletion and global warming potentials
Chemical CFC/HCFC no. Ozone depleting Global warming potential potential
Carbon dioxide 1 Methane 21 Chloroform 4 Nitrous oxide 310 Dichloromethane (methylene chloride) 9 1-Chloro-1,1-difluoroethane HCFC-142b 0.065 2300 Chlorodifluoromethane HCFC-22 0.055 1900 Chlorofluoromethane HCFC-31 0.020 2-Chloro-1,1,1-trifluoroethane HCFC-133a 0.060 Chlorotrifluoromethane CFC-13 1.0 14000 Dichlorodifluoromethane CFC-12 1.0 10600 Dichlorofluoromethane HCFC-21 0.040 1,1,1,2-Tetrafluorochloroethane HCFC-124 0.02–0.04 620 Trichlorofluoroethane (Freon 113) HCFC-131 0.007–0.050 Trichlorofluoromethane CFC-11 1.0 4600 Trichlorotrifluoroethane CFC-113 0.8 6000 1,1,1-Trichlorotrifluoroethane CFC-113 0.80 6000
CFC = chlorofluorocarbon; HCFC = hydrochlorofluorocarbon Adapted from DETR Climate change: draft UK programme (2000)
Environment Agency Guidance on the management of landfill gas 35 3
Gas Management Plan
3.1 Definition ● set out the methods of implementing site-specific gas management systems to: The Gas Management Plan provides a framework for the management of landfill gas based on the site – prevent the migration of and control any characteristics and the nature and extent of the gas release of landfill gas control system. The Plan should provide a clear and – minimise the impact on local air quality auditable route-map setting out the methods, – minimise the contribution to climate change procedures and actions to be implemented at the site – control the release of odorants for the duration of the PPC permit, up to the point of – minimise the risk of accidents surrender. – prevent harm to human health The Gas Management Plan should be prepared either ● set out the installation criteria and construction as a stand-alone document or as part of the quality assurance procedures for the gas control documented site operational details and procedures, measures; e.g. as part of the PPC application. Applications for ● set out the procedures and responsibilities for permits must include ‘the proposed operation, installation, operation, maintenance and monitoring and control plan’ and the Gas monitoring of the gas control measures; Management Plan will be an integral part of any ● demonstrate that performance of the control submission. measures meets the requirements and objectives The risk assessment should be used by the operator for gas management; when developing the Gas Management Plan for a ● set out the procedures for managing changes and particular landfill. The Plan should set out the risk reviewing the performance of the gas control factors and illustrate how these risks are to be system. minimised and monitored. It should be used as a tool to demonstrate that the gas control system is appropriate for the landfill conditions during site 3.3 Framework for the Gas Management development, operation, closure and post-closure Plan stages. The following sections set out the framework for the The Gas Management Plan is a live document and Gas Management Plan. The key elements of the Gas should be reviewed and updated regularly to ensure Management Plan are: that adequate controls are in place to meet identified ● risk assessment (see Chapter 2) standards and objectives. ● control measures (see Chapters 4 and 7) ● operational procedures (see Chapter 7) ● monitoring plan (see Section 3.3.3) 3.2 Objectives ● action plan (see Section 3.3.4) The objectives of the Gas Management Plan are to: ● aftercare and completion plan (see Section 3.3.5). ● bring together all aspects of gas management 3.3.1 Waste inputs considered during the risk assessment and The type of waste deposited at the site will form the proposed operational controls; basis for the estimation of gas production, while the ● provide an estimate of gas production; estimation of the source term will determine the type ● set out performance criteria for the gas control and extent of gas management at the site. Although measures; the principles and many of the techniques to manage ● set out the design objectives and principles for the and control landfill gas are generic across the range gas control measures;
36 Environment Agency Guidance on the management of landfill gas of site categories, individual Gas Management Plans 3.3.3 Monitoring and sampling plan must be based on the site-specific conditions and The monitoring and sampling plan is an integral part circumstances. of the overall Gas Management Plan. It allows the The following should be identified: performance of the gas management system to be established and assessed against the conceptual site ● waste types – the quantities and types of waste to model and provides for developments of the model. be disposed of, and the rates and methods of This offers increased confidence in environmental filling; protection and the ability to demonstrate that the ● landfill gas source – the estimated nature and the objectives of the Gas Management Plan are achieved. anticipated quantity of gas that could be Further details on monitoring and sampling are given generated during each phase of development (i.e. in Chapter 8. operational, closure and post-closure); ● landfill gas risk assessment – this forms the basis of The monitoring and sampling plan should include as the Gas Management Plan as it evaluates the risks a minimum: to receptors. It should be prepared using the ● a schedule for specific data collection and approach set out in Chapter 2. frequency of monitoring at all stages of the site 3.3.2 Control measures (i.e. prior to site development to obtain background data and beyond the closure of the The elements of gas control are: site to demonstrate site completion); ● containment ● a layout showing the construction and location of ● collection monitoring points in relation to the site, ● treatment. surrounding area, geology, and phasing of operation; These are considered as part of the risk assessment ● a description of the measurement techniques and (Chapter 2) and are discussed in more detail in sampling strategy; Chapter 7. The method of controlling landfill gas will ● an analysis and testing schedule; depend on a number of factors, which should be ● a methodology for data storage, retrieval and detailed in the Gas Management Plan. In particular, it presentation; should consider: ● the background and action/trigger values against ● landfill development – details for containment which collected data will be evaluated; (lining and capping) and the phasing of landfill ● the methodology for data interpretation, review development and operation (Chapters 4 and 7); and reporting; ● emission standards – these should be clearly stated ● the means of communicating the results of the based on agreed emission limits and the outcome monitoring and interpretation to the regulator. of the risk assessment (Chapter 2); Monitoring data will be used by the regulator to ● collection system – the plan should describe the verify compliance with the Gas Management Plan measures to collect landfill gas from the waste and the permit conditions. body, including the approach to be taken from initial development of the site through to the 3.3.4 Action plan aftercare stage (Chapters 4 and 7), and should The Gas Management Plan must set out the actions include details of the layout, etc; to be taken by the operator as a result of: ● condensate management – the plan should describe the measures to manage condensate ● any abnormal changes observed in collected from the gas control system (Chapter 7); monitoring data; ● inspection, maintenance and servicing – details ● all identified operational problems or failure of the should be provided, for each element of the gas gas control system established as part of the collection and control system, utilisation/flaring routine inspection or maintenance programme; plant and supplementary processing/treatment ● a reported event, e.g. an odour complaint. equipment (Chapter 7); As outlined in Section 2.5, scenarios should be ● utilisation, flaring and treatment – the plan should established during the risk assessment to determine set out in detail the measures to manage the the actions necessary to manage potential accidents collected landfill gas, including such methods as and failure scenarios that might lead to occurrences supplementary processing, utilisation, flaring, and such as: methane oxidation (Chapter 7). If utilisation is not proposed, this must be justified. A detailed ● migration and release of landfill gas appraisal of the proposed measures should be ● impact on local air quality included in all cases.
Environment Agency Guidance on the management of landfill gas 37 ● release of odorants including the immediate actions required. In the case ● harm to human health. of an emergency, the regulator and other appropriate authorities should be notified immediately. Failure scenarios should be identified for each component of the gas management system and The risk assessment should identify the mechanisms appropriate action values should be assigned to by which such extraordinary events can occur. The specific monitoring locations for elements of the gas emergency actions, procedures and protocols control system. resulting from these should be submitted to the regulator as part of the preparation of the PPC The implementation of appropriate action should be application. considered in conjunction with an assessment of the severity of the event. This is particularly important in The emergency procedures for each identified event the case of emergencies and can be defined by must be included in the action plan, which should setting one or more of the following: clearly define: ● compliance criteria, i.e. specific compliance ● the name of the person/position responsible for requirements such as emissions standards; managing the emergency actions; ● assessment/action criteria – derived values based ● emergency notification and contact procedures on compliance/trigger criteria which form an early (e.g. regulator and emergency services phone warning and/or may instigate additional number and contact names); monitoring or emergency procedures. This will ● assessment parameters for each emergency become a compliance requirement when it is scenario; specified in the permit, e.g. greater than 1 per ● description of actions for each emergency scenario cent methane, above background concentration, (what actions are to be taken and who will in a monitoring borehole; undertake them); ● a systems failure criteria, e.g. accidental ● monitoring requirements (specific monitoring disconnection of gas collection wells; procedures for each emergency scenario); ● an event report criteria, e.g. reported odour ● reporting parameters (what should be reported to beyond site boundary. the parties or persons involved); ● completion parameters (what criteria identify the The plan should set out the procedures and protocols completion of the emergency action); by which the operator will manage the effects of ● procedures for reviewing emergencies and the these events by identifying requirements for: performance of the Gas Management Plan. ● emergency actions – immediate measures to These should be reviewed regularly in conjunction counter extraordinary events, e.g. evacuation of with the regulator. Personnel operating the site buildings; should be trained on how to implement the response ● changes to gas management techniques and to individual emergency scenarios. other operational control measures required to control gas on-site, e.g. installation of additional An example emergency plan is included in gas collection wells; Appendix E, which sets out the hierarchy of actions in ● changes to the strategy for routine monitoring relation to emergency response and notification. using identified monitoring guidance to provide Remedial actions improved data to evaluate the event, e.g. increased perimeter monitoring. Where a deficiency is identified, either via routine monitoring, inspection, maintenance or failure of The action plan should be developed as part of the elements of the gas management system, then structured development and application of the appropriate measures need to be identified. An appropriate Gas Management Plan to the satisfaction appropriate remediation time-scale should be of the regulator. The PPC permit holder must prescribed in the Gas Management Plan. For undertake a periodic review of the action plan in example, it should be possible to commence consultation with the regulator. enhanced monitoring protocols within 24 hours or Emergency procedures and protocols incorporate additional collection wells within 7 days (in the case of sacrificial pin wells). Significant events identified at the risk assessment stage, which result in either an unacceptable level of 3.3.5 Aftercare and completion plan risk or which are an extraordinary occurrence, should Details for the aftercare, closure and completion of be identified as emergency scenarios. Specific the site should be developed as part of the structured procedures should be set out to manage these events, application; where appropriate, they will be replicated
38 Environment Agency Guidance on the management of landfill gas in permit conditions. Before waste disposal activities at the site cease, the Gas Management Plan should be reviewed in order to revise the requirements for landfill gas management during the closure phase. The plan should be based on the same principles as those identified above and should: ● define the measures for continued management and monitoring of landfill gas at the site (including maintenance requirements) following closure; ● indicate how the operator will meet the criteria for surrender of the permit for the site. Guidance (applicable in England and Wales) on this subject is available (Environment Agency, 2004a).
Environment Agency Guidance on the management of landfill gas 39 4
Requirements for gas control
This chapter sets out regulatory requirements for the Guidance is given in the following sections on each control of landfill gas. Details of the specific measures element of gas control. Issues such as design, and techniques for the control of landfill gas are construction quality assurance (CQA) and phasing are presented in Chapter 7. typically common to all elements and as such are addressed collectively in Part C. The measures for containment, collection and treatment should be set 4.1 Introduction out in detail in the Gas Management Plan. In order to minimise the potential impact of landfill gas on the environment (as required by the Landfill 4.2 Gas containment Regulations), it is necessary to actively control landfill gas throughout the whole period for which the site 4.2.1 Principles of gas containment has a PPC permit. The same principles also apply to Site containment is provided by engineered barriers, sites that are regulated and will close under the waste which in combination with active gas collection, management licensing regime. control and minimise the migration and emission of In the context of this document, the key gas control landfill gas. Containment is provided by: measures are: ● engineered lining of the sides and the base of the ● containment site to prevent uncontrolled movement of landfill ● collection gas through the base and sides of the site; ● treatment (i.e. utilisation and flaring). ● engineered capping of the site surface to reduce the rate of direct emission of landfill gas to the The general design and operational requirements for atmosphere and control the ingress of air (as well all classes of landfills are set out in Annex 1 of the as controlling the ingress of rainwater); Landfill Directive, Schedule 2 of the Landfill ● reduction of any in-situ gas pressure by gas Regulations 2002 and Schedule 3 of the Landfill extraction. (Scotland) Regulations 2003. They require the following gas control measures: Further details on the requirements for gas containment are set out in Chapter 7. More information on containment systems, applicable in ● appropriate measures must be taken in order to England and Wales, is given in Guidance on the control the accumulation and migration of development and operation of landfill sites landfill gas; (Environment Agency, 2002f). In Scotland, reference should be made to the Framework for risk assessment ● landfill gas must be collected from all landfills for landfill sites: the geological barrier, mineral layer and receiving biodegradable waste and the landfill the leachate sealing and drainage system (SEPA, 2002). gas must be treated and, to the extent possible, used; 4.2.2 Lining
● landfill gas which cannot be used to produce A number of issues need to be considered when energy must be flared; engineering a landfill liner, which as a minimum, must meet the requirements of the Landfill Directive ● the collection, treatment and use of landfill gas and the Groundwater Directive. For England and must be carried on in a manner, which minimises Wales, the engineering requirements of both damage to or deterioration of the environment Directives are addressed in Landfill Regulatory and risk to human health. Guidance Note 6 Interpretation of the engineering
40 Environment Agency Guidance on the management of landfill gas requirements of Schedule 2 of the Landfill (England and 4.2.3 Capping Wales) Regulations 2002 (Environment Agency, Capping has several benefits, including: 2003c). In Scotland, reference should be made to the Framework for risk assessment for landfill sites: the ● providing encapsulation of the waste geological barrier, mineral layer and the leachate sealing ● reducing leachate formation and drainage system (SEPA, 2002). ● improving the efficiency of gas collection and control Engineering requirements should be decided after ● assisting odour control considering gas and leachate issues, and developing ● reducing the ingress of air the conceptual model in conjunction with site ● reducing surface emissions. investigations and risk assessment to achieve no unacceptable emission/discharge. The lining must restrict the transmission of landfill gas. The minimum The Landfill Directive and Landfill Regulations state that: standards for lining as required by the Landfill Where the potential hazards to the environment indicate Regulations are set out in Table 4.1. that the prevention of leachate formation is necessary, surface sealing may be prescribed. The selection of the artificial sealing liner should be based on the risk assessment, and on the ability of the lining system to contain landfill gas as well as Table 4.2 sets out the guidelines from the Landfill controlling leachate migration and preventing Regulations for surface sealing for each landfill groundwater ingress (where applicable). In most classifications. circumstances, the selection of the artificial lining system is determined by the groundwater risk assessment – although the selection of the landfill Table 4.2 Capping recommendations for lining system should consider landfill gas landfills containment. However, it must be remembered that the permeability of the lining system to gas will be Classi- Inert Non- Hazardous higher than that for leachate fication hazardous
Gas drainage Not Required Not Table 4.1 Lining requirements for landfills layer required required Artificial Not Not Required Classi- Primary Attenuation sealing layer required required fication (sealing) layer Impermeable Not Required Required liner (geological barrier mineral layer required hydraulic permeability) Drainage layer Not Required Required >0.5 m required Inert Not required ≥1mthick ( ≤1x10–7m/s) Soil cover Not Required Required –9 Non- Required ≥ 1mthick ( ≤1x10 m/s) >1.0 m Required hazardous
–9 Hazardous Required ≥ 5mthick ( ≤1x10 m/s) In England and Wales, guidance on the engineering requirements of the Landfill Directive is given in Notes Landfill Regulatory Guidance Note 6 (Environment Where the attenuation layer does not naturally meet the above Agency, 2003c), which states that the Landfill conditions it may be completed artificially and reinforced by Directive recommendations for capping can be other means providing equivalent protection but, in any such case, a geological barrier established by artificial means must be changed on the basis of a risk assessment. As a result, at least 0.5 m thick. the gas drainage layer may not be required for non- hazardous landfills. The above requirements may be reduced to an appropriate extent if, on the basis of an assessment of environmental risks The engineered cap forms an integral part of the gas and having regard in particular to Directive 80/68/EEC: management system and is thus expected to be (a) it has been decided that the collection and treatment of required at all sites that produce landfill gas. The leachate is not necessary; or capping system allows for efficient gas capture in (b) it is established that the landfill poses no potential hazard to conjunction with an active gas collection system. It soil, groundwater or surface water. should be designed to provide maximum gas control compatible with the control of rain and surface water ingress.
Environment Agency Guidance on the management of landfill gas 41 4.3 Gas collection sizing and costing of the required plant. Operators should note that the Landfill Regulations require that The collection of landfill gas is a requirement of the landfill gas must be collected from all landfills Landfill Directive and Landfill Regulations for all sites receiving biodegradable waste and that the landfill accepting biodegradable waste. However, gas gas must be treated and, to the extent possible, used. collection will be required in all landfills where gas Landfill operators will therefore be required to provide production takes place. The type and nature of the the regulator with a full justification if they do not gas collection systems for such sites will depend on propose to utilise landfill gas. the types of wastes and their gas generating potential. The following set out indicative criteria for establishing the viability for landfill gas utilisation To control the potential risk associated with gas (Environment Agency, 2000a). An assessment against migration and emissions, landfill gas must be each of the indicative criteria should be undertaken at removed from the waste, treated and, if possible, the PPC permit application stage to confirm whether utilised. The gas collection system thus forms a key landfill gas utilisation is likely to be required. A similar element of the control and minimisation of the risks consideration is appropriate for sites regulated under from landfill gas, which would otherwise result in waste management licensing. unacceptable impacts. ● Size of site – sites with less than 200,000 tonnes The collection system should be designed to: of waste are unlikely to produce sufficient landfill ● prevent migration gas to generate more than 0.75 MW and are of ● minimise emissions limited commercial interest. However, the ● optimise utilisation (where possible). technology associated with the minimum capacity of utilisation systems is being continually improved The elements of the gas collection system, which and units with a capacity of around 0.3 MW are covered in more detail in Part C, include: available. ● collection wells ● Geometry of site – recoveries of landfill gas are ● collection layers highest on large deep sites and it is often ● collection pipework impractical to recover landfill gas from shallow ● extraction plant sites or from peripheral areas of sites. It is ● condensate management system. generally accepted that the minimum depth of site from which landfill gas can be effectively and These elements should be incorporated at defined efficiently recovered is around 4 metres. stages during the development of the site and set out ● Gas flow rate – a total landfill gas flow rate of in the Gas Management Plan. They may consist of a between 600-750 m3/hour (at 50 per cent range of temporary measures to collect gas until the methane) is required to generate 1 MW. The flow permanent systems can be installed. rate refers to what can actually be recovered from Collection of gas is the primary means of controlling the site. odour. If a landfill gas odour can be detected at the ● Waste composition – if the waste within the site site boundary, then it is an indication that landfill gas contains 75 per cent or more inorganic wastes, is not being collected efficiently. Passive venting is an then landfill gas production from biodegradation unacceptable control measure for landfill gas. will be minimal. Nevertheless, this may still be significant in terms of its potential environmental Further details on gas collection and monitoring are impact. given in Chapters 7 and 8. ● Site location – if a direct use scheme is being considered, then the end user for the gas should be within 10 km of the landfill site; otherwise, the 4.4 Utilisation, flaring and treatment cost of transporting the gas can make the 4.4.1 Indicative assessment criteria for utilisation schemes uneconomic. Before a landfill site can be considered for gas Where utilisation is not possible then landfill gas must utilisation, the operator must provide either proof or be flared in accordance with relevant guidance demonstrate the amount of gas produced by the site (Environment Agency, 2002d). and provide an estimate of the future rate of production over the lifetime of the site. This will determine if utilisation is feasible and will enable the
42 Environment Agency Guidance on the management of landfill gas 4.4.2 Flaring The final stage of processing in the absence of utilisation is thermal oxidation of the landfill gas in a flare. Where it has been demonstrated that utilisation is not viable or where the above criteria cannot be sustained, flaring of the gas must be provided. There are a number of regulatory requirements. ● No ‘open’ flares are to be installed on Agency regulated landfill sites, except for emergency or test purposes. ● Operational ‘open’ flares at landfills subject to permitting or re-permitting under the Landfill Regulations shall be replaced with ‘enclosed’ flares or other techniques offering equivalent performance as they are re-permitted. Where appropriate, this requirement may be included in an Improvement Schedule, for completion within one year of date of issue of permit. ● Operational ‘open’ flares at landfill sites that remain subject to waste management licensing shall be replaced progressively with ‘enclosed’ flares or techniques offering equivalent performance. This replacement programme will be undertaken on a risk basis, for completion as soon as reasonably practicable, as identified by a site- specific Emissions Review. The improvements identified in the Emissions Review shall be completed at all Agency-regulated landfills by 16 July 2009. In addition, enclosed flares that have a stand-by role for utilisation plant, do not need to be monitored as long as: ● the enclosed flare can be shown to be operating within the Agency’s operational standard; ● the enclosed flare is operational for less than 10 per cent of the time (on an annual basis); ● routine monitoring is not identified as necessary within the site-specific risk assessment. Technical guidance on landfill gas flaring (Environment Agency, 2002d) and flare emissions monitoring (Environment Agency, 2004b), sets out the requirements for flare design, operating principles and monitoring. 4.4.3 Treatment In some situations, treatment of landfill gas may be required to meet emission limits for landfill gas engines. Treatment of the gas stream pre- or post- combustion will be a site-specific issue incorporating cost-benefit analysis (Environment Agency, 2004c).
Environment Agency Guidance on the management of landfill gas 43 5
Requirements for monitoring
5.1 Introduction The Landfill Directive and Regulations lay down minimum requirements for landfill gas monitoring This chapter deals with the regulatory requirements (see Table 5.1). for monitoring. The technical aspects of monitoring are described in Chapter 8. The minimum explicit monitoring requirements for landfill gas from the Landfill Directive and Regulations The objectives of environmental monitoring were set relate to: out in Section 2.6 and include: ● the monitoring of gas within the waste (source); ● demonstrating that the performance of the ● the efficiency of the gas extraction system; control measures meet the requirements and ● atmospheric pressure (during borehole/well objectives of the Gas Management Plan for the monitoring at the site). site, and establish a reliable data set; ● creating an information base so that the site can In addition to these minimum requirements, the be adequately characterised and managed depth to water/leachate and the pressure should be throughout its life; included in the monitoring of all wells. ● providing information from which to determine when completion criteria have been met.
The Landfill Directive and Regulations set the following monitoring requirements. ● The operator shall carry out, during the operational phase, the control and monitoring procedures set out in Annex III and Schedule 3 (respectively). ● Where the procedures reveal any significant environmental effects, the operator shall notify the regulator as soon as reasonably possible. ● When it receives a notification of significant adverse environmental effects the regulator shall determine the nature and timing of corrective measures that are necessary and shall require the operator to carry them out. ● The operator shall report at intervals specified by the regulator on the basis of aggregated data, the results of the monitoring and on such matters which the regulator requires to demonstrate compliance with conditions of the landfill permit or to increase its knowledge of the behaviour of the waste in landfill. ● The operator shall ensure that the quality of: (a) analytical operations of control and monitoring procedures; (b) analyses of representative samples taken are in accordance with regulatory requirements. ● Following definitive closure of a landfill, aftercare procedures shall ensure that: (a) The operator remains responsible for the maintenance, monitoring and control for such period as the regulator determines is reasonable, taking into account the time during which the landfill could present hazards. (b) The operator notifies the regulator of any significant adverse environmental effects revealed by the control procedures and takes remedial steps as required or approved by the regulator. (c) The operator is responsible for monitoring and analysing landfill gas and leachate from the landfill and groundwater regime in its vicinity in accordance with Annex III (Schedule 3 of the Landfill Regulations) as long as the regulator considers that the landfill is likely to cause a hazard to the environment. ● Measures shall be taken to minimise nuisances and hazards arising from the landfill through: emissions of odours and dust. ● Gas monitoring must be carried out for each section of the landfill. ● The efficiency of the gas extraction system must be checked regularly.
44 Environment Agency Guidance on the management of landfill gas Table 5.1 Minimum monitoring requirements for monitoring wells and gas wells
Frequency of monitoring in Frequency of monitoring in operating phase aftercare phase
Potential gas emissions and Monthly2,3 Every six months4 1 atmospheric pressure (CH4, CO2, O2, H2S, H2, etc,) 1 These measurements are related mainly to the content of organic material in the waste.
2 Longer intervals may be allowed if the evaluation of data indicates that they would be equally effective.
3 CH4, CO2 and O2 regularly. Other gases as required, according to the composition of the waste deposited, with a view to reflecting its leaching properties.
4 Efficiency of the gas extraction system must be checked regularly.
Table 5.2 Landfill gas component monitoring To ensure the consistency and long-term reliability of requirements monitoring records, gas monitoring programmes should be: Component Monitoring requirements ● targeted to answer specific questions required for Source Collection wells and monitoring wells permit compliance and to provide a robust assessment where specific risks are identified; Emissions Surface and lateral emissions ● designed to deliver minimum statutory and permit Combustion Flares and engines requirements; ● Air quality Odour undertaken by competent personnel; ● robust and fit for the purpose for which they are Meteorological Weather conditions designed, with proper regard to quality assurance and quality control; ● To provide for the requirements of the Landfill interpreted clearly and appropriately on a regular Directive, landfill gas monitoring should be basis so that results can be reviewed and undertaken for each component set out in Table 5.2. understood by non-technical personnel; ● The specific requirements for monitoring of each reviewed against objectives and the conceptual component are given in Chapter 8. The regulator model, and revised accordingly on a regular basis. may require additional monitoring on a site-specific The development of a landfill gas monitoring basis. programme needs to be based on a thorough The monitoring of landfill gas is an essential factor in understanding of the conceptual model of the site the management of any landfill site. A monitoring and therefore on its setting, its geology and the and sampling plan must be prepared and set out potential emission and migration pathways between within the Gas Management Plan (see Chapter 3). the site and receptors. This information, along with The monitoring plan should provide objectives and leachate and groundwater monitoring records, will describe a site-specific programme of monitoring to form an important part of the evidence required for be undertaken at the landfill site. This will determination of completion conditions for the incorporate: surrender of a site permit. The key steps for landfill gas monitoring are shown in Figure 5.1. ● the type of monitoring to be undertaken; ● the methods of monitoring (including detection The frequency of monitoring depends upon a limits, accuracy, etc.); number of factors. These include the following, which ● monitoring locations; are all considerations for the conceptual model: ● frequency of monitoring; ● age of the site ● appropriate action/trigger levels necessitating ● type of waste accepted action; ● geology of the surrounding strata ● appropriate action plans to be implemented ● control measures installed should any levels greater than the trigger levels be ● potential hazard from migrating gas recorded.
Environment Agency Guidance on the management of landfill gas 45 ● sensitivity and proximity of surrounding 5.2 Monitoring at the site preparation phase development and receptors Gathering monitoring data before and during the site ● results of previous monitoring. preparation phase is an important element of the The requirements for monitoring described in development of the conceptual model, the site Section 5.2 are considered minimum standards. The monitoring plan and the evidence that the site meets frequency of monitoring and the parameters to be the completion criteria. It enables background monitored must be based on an assessment of the concentrations to be established – an important risks posed by the site, incorporate the factors listed parameter to consider when undertaking a risk above, and be agreed with the regulator. assessment and setting trigger levels for a site. In addition, background monitoring allows other sources of methane and carbon dioxide to be identified if present. Such other sources of methane and carbon Establish objectives and standards in dioxide include marsh gas and mine gas (see relation to risk Appendix B). The frequency of monitoring should be sufficient to Design monitoring programmes to meet enable the characterisation of seasonal and other objectives environmental influences. Table 5.3 gives details of the minimum monitoring frequencies required during the preparation of a site. Design, install and maintain monitoring structure Table 5.3 Determinands and monitoring frequencies at the site preparation phase
Gather monitoring data Frequency of Parameters to monitoring monitored during the site during the site Compare monitoring data with design preparation phase preparation phase Background objectives to indicate success or failure 12 data sets CH4, CO2, O2 gas levels in representative of Atmospheric monitoring a 12-month pressure boreholes period Meteorological data Respond to any landfill gas impacts /risks Note: Details of the meteorological parameters to monitor for are given in Chapter 8. Figure 5.1 Process of landfill gas monitoring 5.3 Monitoring during site operational and Monitoring data must be reviewed on a regular basis aftercare phases against the objectives of the Gas Management Plan and any changes submitted and notified to the The Landfill Directive and Regulations require the regulator. The monitoring frequency must not be operator of a site to undertake site monitoring during regarded as fixed for any site and examples of where the operational phase of a landfill site. They also it should be varied include: require the operator to undertake monitoring during the closure and aftercare period of the site until the ● where deviations in the concentrations expected regulator deems the landfill no longer likely to cause from the risk assessment are detected during a hazard to the environment. Table 5.4 summarises routine monitoring; the typical/minimum frequencies considered ● the gas control system, or any element of it, has necessary by the regulator for monitoring at the failed or been changed; various types of gas sampling location associated with ● the pumping of leachate has changed and a landfill. leachate levels change within the wastes; ● capping of all or part of the site has taken place; ● new development takes place in the vicinity of the site; ● any emergency events that warrant further monitoring, e.g. landfill gas in buildings.
46 Environment Agency Guidance on the management of landfill gas Table 5.4 Type of sampling location and typical monitoring frequencies for landfill gas
Monitoring Frequency of Frequency of Parameters to be points monitoring during monitoring during monitored operational phase aftercare period
Surface emissions CH4 concentration/flux (i) Walk over survey Annually Annually Atmospheric pressure and temperature (ii) Flux box monitoring Site-specific (1–5 yrs) Site-specific (1–5 yrs) Meteorological data General surface type and condition
Monitoring boreholes Monthly Six-monthly CH4, CO2, O2 (external to the landfill) Atmospheric pressure Differential pressure Temperature Meteorological data
Monitoring wells Monthly Six-monthly CH4, CO2, O2 (within the landfill) Atmospheric pressure Differential pressure Temperature Meteorological data
Collection wells Fortnightly Six-monthly CH4, CO2, O2 Atmospheric pressure Differential pressure Gas flow rate or suction Temperature Gas collection system Annually Annually Composition of raw landfill gas (including (site-specific) trace components) from the extraction line e.g. manifolds and prior to the disposal system
1 Flares Annually Annually NOx, CO, VOCs, NMVOCs
Utilisation plant Annually Annually NOx, CO, VOCs, NMVOCs
Note: Monitoring wells within the landfill are required 5.4 Other guidance to fulfil the requirement set out in the Landfill A number of other documents related to the Regulations to provide representative gas monitoring for management, control and monitoring of landfill gas each section of the landfill. These wells should be provide useful sources of information. These include: sufficient to characterise the landfill gas source. ● 1 Monitoring of landfill gas (2nd edition). Institute of Non-methane volatile organic compounds Wastes Management (1998); ● Risk assessment for methane and other gases from the ground. Report 152. CIRIA (1995); ● Landfill gas development guidelines. Prepared by ETSU for Dti (ETSU, 1996).
Environment Agency Guidance on the management of landfill gas 47 48 Environment Agency Guidance on the management of landfill gas C
Guidance on the management of landfill gas
Part C: Technical considerations
Environment Agency Guidance on the management of landfill gas 49 50 Environment Agency Guidance on the management of landfill gas 6
Landfill gas production and emissions
This chapter reviews in detail the mechanisms by These processes will also occur in wastes containing which landfill gas is generated, its composition, its biodegradable waste. However, the emitted gases will chemical and physical characteristics, and its generally only be found as trace components mixed behaviour in landfills. The chapter also examines the with the methane and carbon dioxide. behaviour of landfill gas within the wider environment and the effects of fugitive and controlled emissions to atmosphere. It is primarily concerned 6.1 Composition of gas from with the gases released by microbial action on biodegradable waste organic materials in the waste, i.e. biodegradation. Mature landfill gas is a mixture predominantly made The Landfill Directive defines landfill gas as ‘all the up of methane and carbon dioxide, and small gases generated from the landfilled waste’. Landfill gas, amounts of hydrogen. It may also contain varying therefore includes gaseous emissions arising from all amounts of nitrogen and oxygen derived from air physical, chemical and biological processes occurring that has been drawn into the landfill. These are within the waste, e.g. microbial production, chemical typically referred to as bulk gases because they are reactions and direct volatilisation. often present at percentage concentrations (see Table 6.1). The composition of emissions arising from predominantly inorganic landfill sites may be very different from biodegradable landfills. Such landfills Table 6.1 Typical range of bulk compounds in need to be considered on a site-specific basis. They landfill gas are not dealt with in detail in this document, but the main principles governing the production of gas from Bulk landfill Typical value Observed maximum such waste are summarised below. gas (%v/v) (%v/v) Gas can be generated from non-biodegradable Methane 63.8 88.0 wastes by the following chemical processes: Carbon dioxide 33.6 89.3 ● corrosion of metals or reactions between # metals, e.g. hydrogen is emitted; Oxygen 0.16 20.9 ● formation of free acidic gases by reaction of the Nitrogen 2.4 87.0# waste with acidic material, e.g. hydrogen Hydrogen 0.05 21.1 cyanide is emitted; ● release of free bases by reaction of the waste Water vapour 1.8 4.0 with alkali, e.g. amines are emitted; (typical % w/w, ● redox reactions within the waste, e.g. sulphur 25°C) oxides are emitted. # Derived entirely from the atmosphere. Volatile substances may be released from the waste Landfill gas will also contain a wide variety of trace by the following physical processes: components. Around 550 trace compounds ● gas stripping with other released gas or water belonging to a variety of chemical groups have been vapour identified in landfill gas (see Environment Agency, ● heat generated in the waste 2002e); these groups are detailed in Appendix A. ● aerosols carrying liquid droplets Together they may comprise approximately 1 per ● dust carrying sorbed materials cent of the gas. Table 6.2 summarises both the variety ● reactions between organic compounds to form and concentration of some trace components of more volatile species, e.g. the formation of landfill gas that have been reported in the UK. esters.
Environment Agency Guidance on the management of landfill gas 51 Table 6.2 Average concentration of a variety of trace components of landfill gas
Name Chemical group Median concentration Average concentration (µg/m3) (µg/m3)
1,1-Dichloroethane Halogenated organics 13,260 476,223 Chlorobenzene Halogenated organics 11,880 246,589 1,1,1-Trichloroethane Halogenated organics 12,905 189,826 Chlorodifluoromethane Halogenated organics 11,570 167,403 Hydrogen sulphide Sulphured compounds 2,833 134,233 Tetrachloroethene Halogenated organics 16,640 112,746 Toluene Aromatic hydrocarbons 11,995 86,221 Chloroethane Halogenated organics 5,190 77,867 n-butane Alkane 13,623 67,412 Chloroethene Halogenated organics 5,600 64,679 Carbon monoxide Carbon Monoxide 5,822 62,952 Ethylbenzene Aromatic hydrocarbons 6,480 37,792 1,2-Dichlorotetrafluoroethane Halogenated organics 3,200 34,046 α-pinene Cycloalkenes 29,300 33,248 cis-1,2-Dichloroethene Halogenated organics 7,700 33,129 Xylene Aromatic hydrocarbons 4,700 23,900 Dichlorofluoromethane Halogenated organics 3,500 20,131 n-hexane Alkanes 5,000 19,850 Dichloromethane Halogenated organics1,240 19,054 n-nonane Alkanes 8,120 19,015 Butan-2-ol Alcohols 5,400 18,704 1,2-Dichloroethane Halogenated organics 1,575 16,495 3-Methyl-2-butanone Ketones 1,984 13,614 Source: Environment Agency (2002e)
Three processes are responsible for the presence of Other ground-based gases may also be rich in carbon these compounds: dioxide (e.g. degassing limestone-rich sediments). ● the volatilisation of chemicals disposed of as These categories of gas are not the central concern of waste; this document, but knowledge of their occurrence ● biochemical interactions occurring within the and composition is important to distinguish methane waste; in landfill gas from other, potentially interfering ● chemical reactions. sources. A number of analytical techniques can be used to discriminate between landfill gas and In addition to landfill gas, there are other sources of methane-rich gas from other sources, including: ground-based gas in the environment which are rich in methane. These gases may be of biogenic or ● hydrocarbon fingerprinting thermogenic origin (see Appendix B) and include: ● analysis of trace components ● radiocarbon dating ● natural mains gas ● isotopic ratio analysis. ● geologically-derived methane (mine gas) ● marsh gas In the event of potential landfill gas migration where ● sewer gas. the situation is complicated by the possible presence
52 Environment Agency Guidance on the management of landfill gas of other methane-rich gases, early analysis using the Aqueous media such as landfill leachate and above techniques should be undertaken. Further condensate can act as vectors for the migration of information on other gas sources is given in dissolved methane. The potential for leachate and Appendix B. condensate to act as a secondary source of methane emissions should be considered as part of the site- based risk assessment. It is particularly important to 6.2 Landfill gas characteristics be aware of the potential for the accumulation in enclosed spaces of methane transported via this 6.2.1 Density route. The density of landfill gas is variable and depends on 6.2.3 Flammability and explosivity its composition. Methane is a flammable gas, with a calorific value of ● A mixture of 10 per cent hydrogen (density 0.08 35.9 MJ/m3, which forms explosive mixtures with air kg/m3) and 90 per cent carbon dioxide (1.98 3 when present between the concentration limits of 4.4 kg/m ), as typically produced during the early o per cent and 16.5 per cent v/v at 20 C and 1 stages of degradation, is denser than air (1.29 atmosphere pressure. These limits are known as the kg/m3). lower explosive limit (LEL) and upper explosive limit ● A mixture of 60 per cent methane (0.72 kg/m3) (UEL) of methane, or the upper and lower flammable and 40 per cent carbon dioxide – typical of a limits, respectively. mature anaerobic landfill gas – is slightly lighter than air. The migration and dilution of landfill gas with air can, therefore, result in the formation of highly explosive Landfill gas is a mixture and, under most conditions, atmospheres. The minimum oxygen content that is its components do not separate into layers. However, required for methane ignition is approximately 14 per the potential for compositional stratification of cent (by volume). The locations identified in the different mixtures in monitoring boreholes must be Section 6.2.4 on asphyxiation are also relevant for the considered in the design of any landfill gas formation of explosive mixtures of landfill gas and air. monitoring plan (see Chapter 3). Landfill gas can also contain variable concentrations 6.2.2 Solubility of other flammable agents including hydrogen Landfill gas contains a range of components that can (flammable limits 4–75 per cent) and hydrogen dissolve in aqueous media (including landfill leachate sulphide (flammable limits 4–44 per cent). Other non- and condensate). These soluble, partially or sparingly flammable components of landfill gas have an effect soluble compounds include bulk and trace on the flammable limits indicated above. The effects components of the gas. of carbon dioxide on the flammable limits of methane are addressed more fully in Appendix C. Under equilibrium, the extent to which a compound enters into solution depends on factors such as: 6.2.4 Asphyxiation ● temperature The accumulation of landfill gas in enclosed spaces ● the partial pressure of the gas/compound can pose a direct risk to humans due to asphyxia. This ● chemical interactions between the compound (the may be caused when the oxygen content of the solute) and the aqueous media (the solvent). atmosphere in the breathing zone is reduced below 10 per cent by volume by admixture with migrating Changes in these factors can also result in degassing landfill gas. of the aqueous medium and the evolution of a gas containing gases/compounds that were formerly in There is evidence that, in extreme situations on solution. Degassing can occur when the liquid landfills with significant depressions, gullies etc., containing the dissolved gases is exposed to changes concentrations of carbon dioxide greater than 5 per in temperature and pressure and/or mechanical cent can accumulate when there is little wind. agitation in plant and pipework used to transport and Voids and enclosed spaces in or near to landfill sites treat leachate and condensate. can be locations that are particularly at risk from Methane is slightly soluble in water (35 ml migrating landfill gas, which poses a potential methane/litre water at 17oC) (O’Neil et al., 2001). explosive or asphyxiant hazard. Such locations Carbon dioxide is more soluble in water, ionising to include manholes, sewers, or tunnels and even poorly form bicarbonate and carbonate ions. This disparity in ventilated spaces below portable buildings. solubility is partially responsible for observed variations in bulk gas composition.
Environment Agency Guidance on the management of landfill gas 53 In areas that are accessible to humans, action is period). Migration of trace gases through the ground needed to prevent the oxygen falling below 18 per to a receptor such as a dwelling without dilution by cent by volume at atmospheric pressure (HSE, 2002). atmospheric air may cause toxicity thresholds to be Where this is not practical, humans should be exceeded. excluded unless they have appropriate personal protection (including breathing apparatus). Table 6.4 Physiological effects from respiration Physiological effects arising from respiration in low of carbon dioxide oxygen environments are listed in Table 6.3. Consentration of Physiological effects Table 6.3 Physiological effects of asphyxiation carbon dioxide (%) 0.03 None, normal atmospheric Oxygen Physiological effects concentration concentration (%) 0.5 Slightly deeper breathing 18 Blood saturation adequate for resting, walking and heavy work 2.0 Lung ventilation increased by 50 per cent 17 Faster, deeper breathing, slight impairment of judgement 3.0 Lung ventilation doubled
16 First signs of anoxia. Dizziness, 5–10 Three-fold increase in rate of buzzing in ears respiration. Rapid exhaustion and headaches 12–16 Increased breathing and pulse rate. Muscular co-ordination 10–15 Intolerable panting. Severe impaired headaches, collapse
10–14 Emotional upset. Abnormal 25 Death fatigue upon exertion
6–10 Nausea, vomiting 6.2.6 Corrosion unconsciousness. Collapse may Some components of landfill gas and its derivatives reoccur with person unable to have a corrosive potential. This potential should be move or cry out considered when designing appropriate gas collection <6 Convulsions, gasping respiration, and treatment systems. Corrosion accelerates wear on death plant and equipment, and reduces their effectiveness as gas control measures. This issue must be addressed as part of a formal risk assessment and suitable site 6.2.5 Toxicity procedures for monitoring (e.g. oil analyses), Some of the constituents of landfill gas, including maintenance and inspection, repair and life-cycle carbon dioxide and a number of trace components, replacement must be developed. These procedures can have toxic effects if present in high enough should be described in the Gas Management Plan. concentrations. Table 6.4 contains information on the Carbon dioxide is soluble in water and will form physiological effects of exposure to carbon dioxide. carbonic acid in aqueous solutions associated with the The trace components of landfill gas do not usually gas. Condensate will be acidic – typically with a pH in represent a health hazard following normal the range 3 to 6.5 – due to dissolved carbon dioxide atmospheric dilution. However, this should be and acidic trace compounds. Such aqueous media demonstrated on a site-specific basis through the can corrode a range of metals. application of a risk assessment. The combustion of landfill gas in utilisation or control Hydrogen sulphide, which may be present in landfill systems will generate substantial quantities of carbon gas, is toxic at low concentrations and will dull the dioxide. Combustion of certain trace components, olfactory senses at higher concentrations such that its such as halogenated or sulphuretted compounds, characteristic odour is no longer detectable. Guidance may give rise to highly acidic and aggressive Note EH40 (HSE, 2002) recommends occupational products. Some of these may corrode equipment or exposure standards of 10 ppm (8-hour time-weighted dissolve in oils and lubricants causing corrosion of average (TWA) reference period) and short-term plant components. At elevated temperatures, carbon exposure standards of 15 ppm (10-minute reference
54 Environment Agency Guidance on the management of landfill gas dioxide, hydrogen, hydrocarbons and water vapour document is directed at facilities regulated under the can cause decarbonisation (removal of carbon from waste management licensing regime, much of the an alloy) and carbonisation (coking) reactions, with guidance is relevant to landfills permitted under PPC. alloys strengthened by the addition of interstitial 6.2.8 Ecotoxicity carbon. The lateral sub-surface migration of landfill gas can Pretreatment of landfill gas prior to combustion can cause damage to vegetation on adjacent land and be used to reduce the corrosive potential of landfill crop die-back (chlorosis). Although plant death in gas. Guidance on gas treatment technologies for landfill many situations has been attributed to root zone gas engines (Environment Agency, 2004c) provides oxygen displacement by the bulk components of additional information. landfill gas, several trace components of the gas are 6.2.7 Odour known to exert phytotoxic effects when borne in sub- surface gas or ambient air. In addition, products of Trace compounds present in landfill gas are landfill gas combustion (e.g. acid gases such as responsible for many of the malodours associated hydrogen chloride, hydrogen fluoride and oxides of with landfilling operations (see Appendix A). Odour nitrogen and sulphur) will produce emissions that are may cause local annoyance and can be responsible potentially damaging to vegetation and ecosystems. for a considerable proportion of the complaints made Efficient gas collection and combustion systems to both landfill operators and regulators. The should provide an effective control mechanism. presence of odour is often linked to concerns about the impact of landfill gas emissions on human health. 6.2.9 Global warming potential Malodorous species can have very low odour Carbon dioxide, methane and a variety of thresholds (i.e. the concentration of a compound in halocarbons found in landfill gas are all greenhouse air that is just detectable to the human nose). gases. This means that they can absorb infra-red Important odorants (see Environment Agency, 2002e) radiation from the Earth’s surface (in the 7–14 µm sometimes reported in landfill gas include: region of the spectrum), which is normally lost to space. This absorption produces thermal energy, a ● hydrogen sulphide; proportion of which is re-radiated back to earth as ● organosulphur compounds, e.g. methanethiol and heat. dimethyl sulphide; ● carboxylic acids, e.g. butanoic acid; Methane is the second most important greenhouse ● aldehydes, e.g. ethanal gas after carbon dioxide. In 1998, the methane ● carbon disulphide. emissions inventory for the UK totalled some 2.6 million tonnes, of which approximately 29 per cent or In complex mixtures such as landfill gas, the presence 0.775 million tonnes were derived from landfills of several odorants may cause additive and hyper- (Defra, 2002b). Landfill gas is therefore a major additive effects (Warren Spring Laboratory, 1980). source of UK methane emissions. Uncontrolled landfill gas emissions can, under certain Landfill gas may also contain CFCs and other meteorological conditions, give rise to odours halocarbons which are ozone-depleting substances extending over several kilometres from site and also contribute to global warming. Combustion boundaries. The emissions of landfill gas may need to of landfill gas can also lead to emissions of nitrous be diluted several million times in order to render its oxide, which has a global warming potential odour undetectable. approximately 300 times greater than that of carbon The availability of such dilution will depend on the dioxide. Further information on global warming prevailing meteorological conditions and the physical potentials is given in Table 2.9. characteristics of the site. Still, calm conditions during 6.2.10 Photochemical pollution cold periods are more likely to give rise to poor dilution. Such conditions (Pasquill’s atmospheric Hydrocarbons present in landfill gas that possess stability categories F and G) generally prevail for more than one carbon atom per molecule can yield about 9.4 per cent of each year in the UK. Persistent highly reactive radicals when exposed to the ultra- odour can result in a number of impacts upon violet radiation present in sunlight. Methane can also amenity and quality of life for neighbouring be ‘activated’ by reaction with hydroxyl radicals. communities. The Agency has produced guidance on Photo-activated hydrocarbons, such as the CH3O• the regulation of odour at waste management radical can react with atmospheric concentrations of facilities (Environment Agency, 2002a), which is nitrogen oxides to produce nitrous oxide (N2O) and, applicable in England and Wales. Although this after a sequence of reactions involving oxygen, tropospheric ozone, producing photochemical smog.
Environment Agency Guidance on the management of landfill gas 55 Photochemical smog is a mixture of chemicals, the fill are replenished by an influx of air, the principally hydrogen peroxide and peroxyacyl-nitrate. concentrations of both of these gases will decay Tropospheric ozone is potentially harmful to humans during the course of Stage 1 activity. This is due to at a concentration of 200 µg/m3 and gives rise to consumption of oxygen and the purging of nitrogen vegetation damage above 60 µg/m3. The from the fill due to the liberation of other gases. The contribution to photochemical pollution is unlikely to presence of percentage concentrations of nitrogen be significant, but may be a localised air quality issue and oxygen in landfill gas, which is collected from that requires consideration as part of the formal risk waste masses that are undergoing degradation in assessment for the landfill. Stages 2–4 of Figure 6.1, is indicative of a failure in the gas management regime. This could reflect excessive suction on parts of the gas collection 6.3 Gas production system, tears in the fabric of pipes and plant or other failures that allow air to enter the collection system. Landfill gas is produced by complex biological and physiochemical processes within the landfill. Stage 2 of Figure 6.1 is associated with the onset of anaerobic conditions within the waste. This is 6.3.1 Methane production characterised by hydrolysis and acetogenic processes Biodegradable organic material present in landfilled mediated by hydrolytic and cellulolytic bacteria waste undergoes microbial degradation. This liberates including species such as Clostridium and Bacillus. This the gaseous intermediates and end-products that activity breaks down large chain polymers present in make up landfill gas. The idealised evolution of these waste (e.g. lipids, proteins and carbohydrates) to components from a single body of waste with time successively smaller molecules. Short chain organic (from the moment of deposit) was described by compounds such as ethanoic acid (acetic acid), Farquhar and Rovers (1973), and is shown in Figure ethanoates (acetates) and ethanol are produced, 6.1. together with ammonia, gaseous carbon dioxide, hydrogen, water and heat. The hydrogen and carbon In Stage 1 of Figure 6.1, oxygen contained within the dioxide produced during Stage 2 continue to purge waste upon deposit is consumed – primarily by the remaining nitrogen from the landfill atmosphere. aerobic microbial activity, which results in the evolution of mainly carbon dioxide, water and heat. Stage 3 of Figure 6.1 is characterised by a period of Unless concentrations of oxygen and nitrogen within transitional anaerobic activity during which
Figure 6.1 Idealised representation of landfill gas generation
Source: DoE, 1995
56 Environment Agency Guidance on the management of landfill gas methanogenesis is initiated and gradually accelerates Methanogenesis is achieved via two principal routes: until a balance with the rates of hydrolysis and ● the acetogenic mechanism, in which short chain acetogenesis is achieved. Methanogenic bacteria are a organic compounds (principally ethanoates) are distinct group of anaerobic micro-organisms, which used as substrate; occupy a crucial niche in anoxic ecosystems and play ● the lithotrophic mechanism, which utilises an important role in the anaerobic degradation hydrogen and carbon dioxide. sequence for landfilled wastes. They utilise substrates (ethanoate, hydrogen and carbon dioxide) produced In Stage 4 of Figure 6.1, the rates of by hydrolysis and acetogenic bacteria to produce hydrolysis/acetogenesis and methanogenesis are in gaseous carbon dioxide and methane. Importantly, relative equilibrium. This provides steady state this provides a route for terminal interspecies conditions during which methane and carbon dioxide hydrogen transfer and electron removal, which allows are evolved in the ratio of about 3:2. Figure 6.2 the microbial and anaerobic degradation of organic details the major steps in the decomposition of material to proceed to completion. biodegradable waste to form landfill gas.
All solid wastes Organic waste Inorganic waste
I Aerobic
Cellulose Proteins polymer Lipids Inorganic salts
Hydrolysis (Aerobic)
H 2 O II Acidogenic Hydrolysis and Fermentation (Anaerobic) Sulphate reducing Metal sulphides bacteria Soluble intermediates eg volatile acids, ammoniacal nitrogen Liberated anions eg nitrate H 2 S
III Acetogenic Acetogens CO2 H2
Acetate Formate
Homoacetogenic bacteria
IV Methanogenic
(Methanogens) CH 4
V Aerobic Methane oxdising organisms
Key CO2 H 2 O Process Product
Micro-organism
Figure 6.2 Major steps in the decomposition of biodegradable waste to form landfill gas (adapted from DoE, 1995)
Environment Agency Guidance on the management of landfill gas 57 Stage 5 of Figure 6.1 represents a period of sulphate-reducing bacteria and the liberation of endogenous respiration during which the organic hydrogen sulphide. The sulphate may originate from substrate required for microbial activity becomes animal or vegetable materials (e.g. fermented grains limited. At this time, the composition of the and food processing waste) but, in most recorded interstitial gases within the fill gradually assumes that circumstances, arise from industrial wastes containing of atmospheric air. large amounts of calcium sulphate (gypsum) such as plasterboard, gypsum quarry spoils and filter cakes In practice, the idealised profiles described by the rich in calcium sulphate (Young and Parker, 1984). model (Figure 6.1) are rarely achieved. Varying Sulphate reduction is one of a suite of competing degrees of phase overlap, phase omission and, even, processes that occur within the landfill environment. temporary cessation have been reported from the Iron reduction – from Fe(III) to Fe(II) – is one of a field. In addition, the duration of particular phases number of such reactions that are energetically and the overall length of time taken for a body of favourable to sulphate reduction. Consequently, the waste to pass through the full degradation sequence availability of ferric ions could potentially inhibit the varies considerably from one site to another. This onset of sulphate reduction. reflects the influence of a wide range of factors, which are discussed in more detail in the following Sulphate-bearing wastes should not be deposited in sections. landfill cells that are also used for the deposit of biodegradable wastes. Leaching levels for sulphates 6.3.2 The formation of hydrogen sulphide are laid down by European Council Decision of 19 Under the anaerobic conditions encountered in December 2002 establishing criteria and procedures landfills, soluble sulphate salts can undergo for the acceptance of waste at landfills pursuant to microbially mediated reduction (see Figure 6.3) to Article 16 of and Annex II to Directive 1999/31/EC produce hydrogen sulphide and other sulphur- (Council of the European Union, 2003). The exclusion containing odorous compounds. These reactions are originates within Section 2.2.3 of this Decision mediated by a specific group of bacteria known as concerning gypsum waste: sulphate-reducing bacteria, which have been Non-hazardous gypsum-based materials should be identified in the landfill environment. Hydrogen disposed of only in landfills for non-hazardous waste sulphide is an odorous, potentially toxic and in cells where no biodegradable waste is accepted. flammable gas. The limit values for TOC and DOC given in sections Sulphate may also be reduced chemically and 2.3.2 and 2.3.1 shall apply to wastes landfilled microbially under anoxic conditions to produce together with gypsum-based materials. insoluble metal sulphides. These reactions probably 6.3.3 Compositional variations in landfill gas explain why, in many landfills accepting inputs of household and commercial wastes, hydrogen sulphide The composition of landfill gas will vary from one site is not found in high concentrations in landfill gas. to another, from one cell of a landfill to another, and will change over time. Some of these changes can be However, hydrogen sulphide can be generated from attributed to: landfills in which large amounts of sulphate-based wastes have been deposited. The ready availability of ● differences in waste composition, pretreatment soluble sulphate results in the increased activity of and storage; ● changes in the rate and predominant form of microbial activity, e.g. aerobic/anaerobic; ● the age of the emplaced wastes; Carbon source Sulphate + water ● gas management regime; ● the hydraulic characteristics of the site; ● the physiochemical properties of waste Flavoproteins ATP lost components; Electrons ● the differing properties of the components of ATP produced generated ATP landfill gas, e.g. solubility; generated ● landfill temperature. Temporal variations in landfill gas composition will Carbon dioxide Sulphide + hydroxyl ions reflect the long-term degradation of the waste, and + water + acetate short-term changes in site characteristics and environmental factors. Some of the most significant Figure 6.3 Dissimilatory sulphate reduction long-term trends include: (adapted from Scott et al., 1998)
58 Environment Agency Guidance on the management of landfill gas ● initial increase in the total concentration of trace Waste composition compounds released during early phases of landfill The composition of the waste deposited within a activity (including a high concentration of landfill will influence both the rate of production and halogenated hydrocarbons); the composition of the landfill gas generated. ● rapid depletion in the concentration of highly Inorganic waste landfills will yield landfill gas at a rate volatile CFCs and low molecular weight alkanes; and with a composition that differs significantly from ● pronounced reduction in the concentration of that generated by biodegradable waste landfills. alcohols and esters as methanogenic activity is firmly established; The biodegradable fraction of waste is the portion, ● accumulation of high molecular weight aliphatic which under landfill conditions can undergo microbial and aromatic compounds over time under degradation to produce gas and liquids. Currently 60 methanogenic conditions; per cent of the municipal waste produced in the UK is ● overall decrease in the total concentration (but believed to be biodegradable waste. not necessarily mass flux) of trace components Article 5.2 of the Landfill Directive sets three over time once methanogenic activity is progressive targets to reduce the amount of established and progresses with time. biodegradable municipal waste sent to landfill. The The composition of landfill gas can also vary within magnitude of the reduction and required schedule for gas extraction and collection systems due to the UK is shown in Table 6.5. admixture with air and gas/condensate and other Table 6.5 UK National target to fulfil Article 5.2 interactions. of the Landfill Directive The migration of landfill gas through sub-surface strata can also affect composition through physical Objectives (e.g. adsorption), chemical and biological (e.g. By 2010, biodegradable municipal waste (BMW) to landfill methane oxidation) interactions between the gas and must be reduced to 75 per cent of the total BMW (by the surrounding rocks and minerals (lithology). These weight) produced in 1995. processes can alter the relative concentration of methane and carbon dioxide, and the trace chemistry By 2013, BMW to landfill must be reduced to 50 per cent of the gas as it moves further from the landfill source. of the total BMW (by weight) produced in 1995. They can also promote changes in the ratio of stable By 2020, BMW to landfill must be reduced to 35 per cent isotopes present in the gas with migrated distance. of the total BMW (by weight) produced in 1995. Ward et al. (1992) reported such isotopic fractionation, where the methane associated with a Source: DETR, 2001 sub-surface plume of landfill gas became isotopically heavier with migration distance while the carbon The biodegradable fraction of the municipal waste, dioxide in the plume became isotopically lighter. which can produce landfill gas, is primarily made up These changes were associated with methane of cellulose and hemicellulose – although not all the oxidation bacteria, which preferentially metabolised cellulose in waste is available for biodegradation. 12C rather than heavier carbon isotopes (13C and 14C). Table 6.6 shows the principal landfill gas generating These potential variations in landfill gas composition components of waste. Table 6.6 indicates that should be considered when assessing monitoring approximately 25–30 per cent by weight of municipal results from off-site boreholes and in investigations waste actually degrades to produce landfill gas. concerning migrating landfill gas. However, this fraction will be different for sites 6.3.4 Factors controlling gas production accepting a high proportion of industrial and other commercial wastes. Landfill gas production is typically affected by: The overall biodegradable fraction of waste disposed ● waste composition at landfills will progressively reduce as the ● density of waste requirements of the Landfill Directive are met. This ● moisture content and distribution may result in changes to the composition and yield of ● pH and nutrient availability landfill gas in the future. However, the proportion of ● temperature biodegradable waste accepted by any one site and its ● presence of toxic agents and chemical inhibitors. consequent contribution to the source of landfill gas These are considered in the following sections. must be considered on an individual basis. This forms Further information can be found in the research part of the source term definition required as part of report CWM 039/92 (DoE, 1992). the formal risk assessment for the landfill.
Environment Agency Guidance on the management of landfill gas 59 Table 6.6 Principal landfill gas generating components in waste
Domestic Civic Com- Sewage Typical Cellulose Hemi- De-comp Percentage amenity mercial sludge water (%) cellulose position of biode- Waste Fraction % w/w % w/w % w/w % w/w content (%) (%) gradable % material in waste fraction PAPER/CARD Newspapers 11.4 10 10 30 18.5 9.0 35 20 Magazines 4.8 11 30 42.3 9.4 46 24 Other paper 10.1 50 30 87.4 8.4 98 94 Liquid cartons 0.5 30 57.3 9.9 64 43 Card packaging 3.8 30 57.3 9.9 64 43 Other card 2.8 30 57.3 9.9 64 43 TEXTILES Textiles 2.4 3 25 20 20 50 20 MISC. COMBUST. Disposible nappies 4.3 20 25 25 50 25 Other 3.6 20 25 25 50 25 PUTRESCIBLE Garden waste 2.4 22 65 25.7 13 62 24 Other 18.4 15 65 55.4 7.2 76 48 FINES 10mm fines 7.1 15 40 25 25 50 25 SEWAGE SLUDGE Sewage sludge 100 70 14 14 75 21
Source: Environment Agency, 1999a
moisture content is normally associated with high rates of gas production, although rates do decline as Density of waste saturation is approached. Moisture content can Waste density is a function of the waste deposited, its promote methanogenesis in a number of ways: particle size and the degree of compaction. ● water may prevent methanogenic inhibition by Theoretically, the landfill gas yield per unit volume diluting the toxic products of acidogenesis; increases with waste density. However, increased ● the dissolution and transport of soluble substrates waste densities generally reduce waste permeability, and nutrients required for methanogenic activity thereby inhibiting the free movement of the soluble may be promoted by increasing moisture content; nutrients required by bacteria. Hence, highly ● water may facilitate bacterial transport within the compacted waste at the base of a deep landfill may waste; have a relatively low rate of production. The ● water will aid mixing and buffering within the mechanical characteristics of the waste deposit may landfill system. also influence the efficiency with which landfill gas can be extracted from the landfill. Food and garden wastes within the municipal waste stream usually have a high moisture content, typically Moisture content and distribution of the order of >25 per cent weight. Rainfall, surface Moisture content is one of the most significant factors water infiltration and the products of waste influencing landfill gas production rates. A high breakdown can provide additional moisture.
60 Environment Agency Guidance on the management of landfill gas Moisture content can be distributed throughout the The hydraulic retention time (HRT) of leachate in a waste to a certain extent by recirculating leachate. landfill is typically of the order of several decades (Knox, Recirculation also serves to flush out the products of 1990). Gas retention times (GRTs) are usually orders of degradation. Figure 6.4 shows the gas production magnitude smaller – typically 2–4 weeks at gas rates achieved in landfill test cells at the Brogborough generation rates of 5–10 m3/tonne/year and a waste landfill by liquid injection (recirculation) in density close to 1 m3/tonne. If gas generation rates are comparison with other factors. The potential for stimulated by recirculation or other methods, the GRT process inhibition should be considered when may fall to just a few days. Only in the very late stages designing a leachate recirculation system; unless of landfill stabilisation will gas emission rates fall to the leachate treatment is part of the recirculation system, point where the GRT is determined by external factors the build-up of inhibitory concentrations of such as diffusion rates. Throughout most of the life of a components in the leachate is a possibility. The use of landfill, gas quality therefore reflects more current leachate recirculation in relation to accelerated gas processes. In contrast, leachate composition may reflect production is also discussed in Section 6.3.6. processes that occurred years or decades earlier. Total gas quantity is mainly a function of the waste Gas production within the saturated zone can be types and their degradable carbon content, although inhibited; thus, a high leachate head can result in poor the rate of decomposition depends on site-specific gas production rates compared with similar wastes that factors. The time taken for waste to decompose will are moist but unsaturated. A high head of leachate directly influence the period over which landfill gas and/or perched leachate tables within landfill sites can will be generated at a particular site and thus the also interfere with gas collection and make effective gas period over which control is required. management difficult. Excess moisture within the landfill mass and the pH and nutrient availability subsequent dissolution of soluble materials results in the Waste degradation processes occur under a wider production of landfill leachate. Leachate quality is a range of pH conditions than methanogenesis, which function of the same decomposition processes that proceeds optimally between pH 6.5 and 8.5. Acidic generates landfill gas. However, other waste conditions resulting from the rapid degradation of components can also contribute to leachate biodegradable wastes and an accumulation of composition and leachate quantity is influenced by breakdown products may inhibit or delay methane other factors such as location, geology, hydrology and generation – unless this is buffered by other site engineering.
Figure 6.4 Brogborough: cumulative landfill gas production for all test cells Source: Environment Agency. 1999b
Environment Agency Guidance on the management of landfill gas 61 components of the waste stream. This can ultimately factors including moisture content and the result in the process of ‘acid souring’ where pHs fall composition of the deposited wastes. In situ below the optimum range in which methanogenesis generation rates in UK landfills are typically in the occurs. In addition, a low pH may promote the order of 10 m3 landfill gas per tonne of waste per dissolution of metal ions within the waste mass, year. These rates are equivalent to carbon turnover which may inhibit methanogenic activity. rates of approximately 7–14 g total organic carbon (TOC)/m3/day. Low metal concentrations may produce a leachate that is poorly buffered against the carboxylic acids so Total methane emissions from UK landfills have been that pH falls too quickly for methanogenic estimated to be approximately 775,000 tonnes/year populations to reach a self-regulating size. (Netcen, 2000). This was equivalent to about 29 per cent of the total methane emissions in the UK in Under the equilibrium conditions associated with 1998. At a methane content of 60 per cent v/v, such Stage 4 of the model shown in Figure 6.1 and where emissions would be equivalent to 4.9 million m3 methanogenic bacteria are firmly established, an methane per day or approximately 200,000 m3 optimal pH of about 7.4 will generally be maintained. methane per hour. Temperature Many models have been developed both for Temperature is an important factor influencing the predicting gas production over the lifetime of rate of landfill gas production. During the initial individual landfills and for estimating national aerobic phases of waste degradation (Stage 1 of the contributions to global emissions. Reviews of different model shown in Figure 6.1), temperatures as high as approaches to modelling have been undertaken by 80–90ºC can be encountered. In the majority of Pacey and Augenstein (1990) in the USA, Sterritt landfills, temperatures thereafter will subside, (1995) in the UK and Gendebien et al., 1992 in stabilising at an optimum of 35–45ºC once Europe. A commonly used approach in the UK for methanogenesis is well established. individual landfills is based on that described by Coops et al. (1995). Recent approaches to estimating Shallow landfills may be more sensitive to climatic the UK’s national methane emissions from landfills are conditions than deeper ones and landfill gas reported by DETR (1999) and Defra (2003). production will tend to drop below 10–15ºC. This may result in a seasonal pattern of waste The most common means of estimating rates of gas decomposition and gas production. Deeper landfills production used for individual landfills in the UK is a can have temperatures over 60ºC, with 40–45ºC first-order kinetic model (i.e. exponential decline), common in the first five years after landfilling. with no lag or rise period, and with waste fractions categorised as being of rapid, medium or slow Landfill gas fires may modify the composition of degradability (see Box 6.1). This equation (or similar landfill gas due to the release of volatile compounds. first-order equations) is commonly used in The incomplete combustion of solid and gaseous combination with waste input predictions to produce components within the landfill will also release a gas generation profile for the lifetime of the site. atypical compounds into the landfill gas. Such a multi-phase, first-order decay equation forms Presence of toxic agents and chemical inhibitors the core of the GasSim model (Environment Agency, 2002b), which is described in Section 2.4.4. Methanogenesis can be inhibited completely or partially by chemical agents (Environment Agency, 6.3.6 Accelerated gas production 2000c). Some of these chemicals may be present in The rate of landfill gas production at a site can be household waste deposited at landfills such as accelerated to some extent by the controlled commercial disinfectants and cleaning materials. modification of factors that influence waste Compounds shown to have an effect include degradation within the landfill environment. This may chloroform, chloroacetate, formaldehyde, nitrate, be carried out to enhance rates of site stabilisation or ferric iron, sodium hypochlorite, hydrochloric acid, increase the commercial viability of gas utilisation. hydrogen peroxide, methanol and sulphate. The However, it must be carefully controlled so as not effects of these inhibitors on gas generation may be cause an adverse impact on other aspects of site significant locally within a site. emissions such as odour. 6.3.5 Rates of production and emission By optimising conditions for landfill gas production, Gas production rates may vary substantially from one laboratory studies have achieved very high rates of site to another and even between areas of a single degradation and gas production (with short-term landfill, reflecting differences in rate controlling peaks of up to about 800 m3 of landfill gas per tonne
62 Environment Agency Guidance on the management of landfill gas of waste per year). Test cell research undertaken in degradation products. This can be achieved by the UK and elsewhere using various techniques to recirculating leachate extracted from the waste. enhance rates of landfill gas production has achieved A moisture content of 40–80 per cent is considered rates equivalent to between 20 and 140 m3 of landfill necessary for optimum levels of landfill gas gas per tonne per year (see Figures 6.4 and 6.5). production (Environment Agency, 1999b). Figure 6.5 As previously described, the most common method shows the gas recovery profiles from two of accelerating gas production rates is to enhance the experimental test cells, one with the recirculation of moisture content of the waste mass and flush out the leachate (Cell 1) and the other without recirculation (Cell 2). These test cells (Landfill 2000) demonstrated Box 6.1 Estimating rates of gas production from a doubling of gas production as a result of leachate UK landfills recirculation.
n Enhancement of landfill gas generation rates can also -k t be accomplished by microbial seeding (e.g. by αt = 1.0846.A.Ci.ki.e i Σ adding sewage sludge cake). However, some of the i=1 benefits of this treatment may be attributable to the Where: increase in moisture content that this produces.
3 αt = gas formation rate at time t in m /year A = mass of waste in place in tonnes 6.4 Landfill gas emissions Ci = carbon content in fraction ‘I’ in kg/tonne Landfill operations have the potential to release a
–1 ki = rate constant for fraction ‘i’ in year range of pollutants to the atmosphere. It is essential
–1 that the emission of these pollutants to the k1 = 0.185 year (fast) environment is controlled and managed in a way that –1 k2 = 0.100 year (medium) is consistent with the PPC permit or an existing –1 k3 = 0.030 year (slow) licence. Gaseous emissions can arise from a wide range of sources including: t = time elapsed since deposit (years) ● freshly deposited wastes; n = the number of iterations ( from 1 until n = t ) ● uncapped wastes; Source: Coops et al. (1995) ● caps or temporary cover materials;
60
50
40
30 /tonne/year) 3
(m 20 Landfill gas flow rate
10
0 0 0.5 1 1.5 2 2.5 3 3.5 4 Years since waste emplacement
Cell 1 (recirculation) Cell 2
Figure 6.5 Gas recovery profiles from the landfill 2000 test cells Source: Environment Agency. 1999b
Environment Agency Guidance on the management of landfill gas 63 ● intrusive engineering work and excavation; 6.4.1 Surface emissions ● leachate and the infrastructure for leachate Surface emission rates (fluxes) of landfill gas from collection and treatment; landfill sites have been reported extensively ● cracks, gaps, fissures and along the edges of the (Environment Agency, 1999a; 2004d; Meadows and site capping; Parkin, 1999; Johnston et al., 2000). Figure 6.6 shows ● lateral migration through surrounding geology; methane flux rate data taken from the literature for a ● landfill gas flares and engines (utilisation plant); range of landfill sites with different capping and gas ● emissions through leakages in gas collection and management controls. distribution pipework, e.g. poorly sealed and balanced collection wells in which gas pressure Figure 6.7 summarises known emission rates through exceeds the available suction. the landfill surface and associated capping and Operators must be aware of the potential impacts engineering infrastructure, and the likely dominant associated with emissions to atmosphere. A number flow mechanism. of the components present in landfill gas can give rise to air quality and/or odour impacts. Three of the most significant emissions sources that may be associated with landfills are: ● landfill gas flares ● surface emissions from capped and temporary capped areas ● landfill gas engines. The Agency has produced emissions guidance to cover these sources (Environment Agency 2004b; 2004d; 2004e) and, in conjunction with this guidance, has established emission limit values for them.
10000
1000
100
10 /day 2 1 g/m
0.1
0.01 Key Sites with gas control 0.001 Sites where controls not known Sites without gas control
Figure 6.6 Methane flux rates reported from a range of landfills
64 Environment Agency Guidance on the management of landfill gas Surface or engineering General range of methane flux1 (mg/m2.s) feature <10-4 10-3 10-2 10-1 1 10 100
Intact engineered cap with active full site gas collection Edge of site
Open slope or settlement cracks Gap in the capping layer
Re-laid surface over gas transmission pipework Operational area
Soil cap only
Area adjacent to gas well Open wells, passive vents, open leachate sumps
Figure 6.7 Surface emissions from areas with diffferent surface or engineering features
1 1 mg/m2.s is equivalent to approximately 50.4m3/ha.h at standard temperature and pressure
Total yearly fluxes from individual landfills were is in preference to flow through a basal and lateral estimated by Allen et al. (1997) on the basis of gas liner. Only when a cap is installed on an unlined abstracted. If there is no gas collection, over 90 per landfill will lateral emissions dominate. This is due to cent of the methane generated by a landfill can be the permeability differences between the waste and lost through surface emissions and the rate of the liner, and the relative thickness of the liner. emission on poorly capped sites can match that of 6.4.2 Lateral emissions generation. Under such circumstances, it can be estimated that non-methane volatile organic Table 6.7 shows possible pathways for lateral compound (NMVOC) emissions will be proportional migration of landfill gas. Rates for lateral emissions of to methane emissions. But, because of the higher landfill gas have seldom been measured, but molecular weights and other factors (e.g. higher diffusion-dominated flow is likely to occur over a boiling points), emissions of the heavier, semi-volatile similar range of values to surface emissions (Figures NMVOCs will be subject to the additional effects of 6.6 and 6.7). Lateral emissions dominated by soil vapour pressure and Henry’s Law partition advective flow may occur at higher rates. These rates coefficients (Eklund, 1992). will generally reflect the pressure differential over the pathway and the co-efficient of permeability to gas of Modelling landfill gas emissions using HELGA the migration pathway, which in turn may be (Environment Agency, 1999c) – the precursor model influenced by the mechanical characteristics of the for GasSim – showed that the bulk of landfill gas waste deposit, its degree of saturation and the produced within the waste will generally flow surrounding geology. through the cap, even if this is well engineered. This
Environment Agency Guidance on the management of landfill gas 65 – can provide a significant driving force if the slight Table 6.7 Emission pathways for lateral positive pressure within a landfill (compared with emissions atmosphere) is only a few kPa (millibars). This effect is much more dramatic if the rate of change of pressure Lateral engineering feature or defect is fast, i.e. it takes place over a few hours than if it Natural or engineered clay liner takes place over a few days. Geomembrane or composite liner with CQA defects No liner, clay geology Box 6.2 Representation of slow advective flow No liner, geology with matrix controlled permeability u = K. ∆P No liner, geology with fracture controlled permeability Where: Landfill with lateral but no basal liner u = mass flux of gas (m3 gas/m2.s)
Vent trench or cut off wall with welded polyethylene 2 membrane K = gas permeability of the medium (m /s.Pa) Vent trench or cut off wall with lapped polyethylene ∆P = pressure gradient (Pa/m) membrane
Vent trench or cut off wall with no polyethylene This was exactly the situation that occurred at Loscoe, membrane Derbyshire, in March 1986, where the sub-surface Emissions of landfill gas through the surface and migration and accumulation of landfill gas caused an boundaries of a landfill site are driven by explosion that destroyed a domestic property. concentration differentials (diffusive flow according to Modelling Loscoe ‘after the event’ led Young (1990) Fick’s Law), or pressure differentials (advective flow), and Young et al. (1993) to conclude the following or both. Advective flows are promoted by pressure qualitative guidelines. differentials between locations. Slower, diffusional ● When the atmospheric pressure is steady, the rate flow will still exist in these situations, but flow will be of gas venting is constant and is independent of predominantly advective. However, the impact of the the atmospheric pressure. slow diffusional flow of landfill gas into confined ● When the atmospheric pressure is rising, the rate spaces should not be underestimated. of venting decreases by an amount proportional It may take many years for the gas diffusing into a to the rate at which the pressure is rising. ● confined space to achieve the same concentration as When the atmospheric pressure is falling, the rate that achieved by an advective plume in a few days. of venting increases by an amount proportional to However, both are equally significant, as the build-up the rate at which the pressure is falling. ● of gases can lead to potentially harmful situations. The composition of vented gas can exhibit large fluctuations even though the internal gas The probability of detecting a transitory advective generation rates and compositions are steady. plume is low and routine monitoring may only ● Although the effects are complex, the size of the demonstrate transitory anomalies at monitoring changes are likely to increase as the moisture points. content of the site and the migration pathway Advective gas migration can be caused by rapid increases, and as the pH of the leachate and/or differential pressure changes such as the passing of a groundwater rises. ● low pressure weather system over the landfill site, The size of the short-term compositional coupled with a highly permeable migration pathway fluctuations will increase as the gas generation (e.g. a fracture or conduit). It can also be caused by rate decreases, i.e. older sites with a relatively changing liquid levels in the site or by the rapid relief small residual amount of biodegradable waste are of pressure, which has built up behind a gas barrier especially vulnerable to this phenomenon. ● (e.g. a clay liner). Trace gases show the same variability as bulk gases. Slow advective flow can be represented by Darcy’s ● Within a site, the compositional changes decrease Law, which gives an empirical relationship between with distance from the surface. the pressure gradient and the gas velocity for slow ● It is impossible to gauge the state of degradation flow. Box 6.2 outlines this relationship. of the waste in a site from measurements of gas It is evident from this relationship that a large and rapid composition taken from shallow monitoring points drop in atmospheric pressure – say 3 kPa (30 mbar) at a single time or at infrequent intervals.
66 Environment Agency Guidance on the management of landfill gas ● Practical experience has shown that the critical The emissions from combustion systems can contain rate of fall in pressure is approximately 0.5 kPa compounds that are: (5 mbar) per 3 hours for at least 3 hours. Statistics ● derived from an unburnt fraction of the gas; based on UK monitoring records over the ten ● products of complete combustion; years from 1982 to 1991 show that this type of ● products of incomplete combustion; pressure event occurs about six times per year ● contaminants from lubricants and materials used over the UK. in the gas extraction and utilisation system and Recent analysis of atmospheric pressure drops at the their combustion products; Building Research Establishment (Hartless, 2000) ● contaminants present in the air used in showed that nearly three-quarters of the top 10 per combustion; cent of falls (≥0.35 kPa (3.5 mbars) had a mid-point ● products of pyrosynthesis and pyrolysis during atmospheric pressure greater than or equal to 100 combustion. kPa (1,000 mbar). Landfill gas monitoring should not, The main groups/compounds of emissions associated therefore, be restricted to periods below 100 kPa with landfill gas combustion systems are discussed (1,000 mbar). Analysis of a 35-year atmospheric below. pressure data set at Ringway Airport (Manchester) (Young et al., 1993) suggests the likely occurrence of Carbon dioxide (CO2) is a component of raw an atmospheric pressure fall equal to or greater than (unburnt) landfill gas and is a product of its complete that experienced at Loscoe is once every 28 years. combustion. It will generally be measurable at percentage concentrations by volume in both the 6.4.3 Point source emissions landfill gas and the emissions from combustion plant. Point sources of raw landfill gas emissions will include The concentrations measured in the emissions from leaks from the gas collection system, monitoring combustion plant will generally be lower than those wells, leachate wells and leachate holding/treatment measured in the raw landfill gas. This is due to its facilities. The actual rate of emissions of landfill gas dilution with primary and secondary air introduced from these sources is difficult to quantify without during the combustion process. measurement of specific cases. Minimisation of such Carbon monoxide (CO) is a product of incomplete point sources must be carefully managed through combustion and may be monitored as a measure of regular inspections and routine maintenance as set combustion efficiency in landfill gas utilisation and out in the Gas Management Plan. flaring systems. Carbon monoxide has been measured 6.4.4 Utilisation and flaring plant emissions at relatively moderate concentrations in the emissions from some landfill gas utilisation plant (Environment During combustion processes, fuel is admixed and Agency, 2004e). reacted with oxidant to produce heat and visible radiation. In landfill gas utilisation and flaring systems, Nitrogen oxides (NOx) are combustion products landfill gas is used as the fuel and air used as the derived from the oxidation of nitrogen and nitrogen- oxidant (containing approximately 21 per cent containing compounds during combustion. The oxygen). The stoichiometric ratio of air to methane principal oxide of nitrogen produced during for idealised combustion is 9.52:1 with the basic combustion is nitric oxide (NO), but this may be combustion reaction:
CH4 + 2O2 + 7.52N2 CO2 + 2H20 + 7.52N2 + Heat + Light
If more oxygen is supplied in the fuel air mixture than further converted to nitrous oxide (N2O), particularly is required for stoichiometric combustion, then the during low temperature combustion; the mixture is mixture is termed lean and oxidising. If less oxygen is known as NOx. Due to oxidation by atmospheric supplied than is needed for stoichiometric oxygen, NOx is converted to nitrogen dioxide (NO2) combustion, then the mixture is rich and reducing. over a period of time, once emitted into the air. The latter condition will result in incomplete When quoting concentration data for the oxides of combustion and the formation of intermediate nitrogen, the convention is to express them combustion products such as carbon monoxide and collectively as NOx. NMVOCs (Environment Agency, 2004e). The three potential sources for the derivation of NOx Incomplete combustion may arise from lean mixtures in landfill gas combustion systems are: where insufficient turbulence is provided to fully mix ● oxidation of nitrogen present in the air drawn into the fuel and oxidant, and where the excessive the plant during the combustion process; addition of air cools parts of the combustion zone. ● oxidation of nitrogenous species (including nitrogen) present in the fuel;
Environment Agency Guidance on the management of landfill gas 67 ● reactions between nitrogen and hydrocarbon exhaust emissions are believed to derive from a number radicals in hot exhaust gases. of potential sources, including:
● Higher concentration of NOx will tend to be found in residual unburnt fuel the emissions from landfill gas combustion plant when ● incomplete combustion temperatures are higher and the combustion is more ● synthesis reactions within the hot exhaust gases. efficient. A number of the compounds detected in the emissions Oxides of sulphur are oxidation products from the from utilisation plant have not been reported as combustion of fuels containing sulphur. Landfill gas can components of raw landfill gas used as fuel, suggesting contain a variety of sulphur-bearing species including a thermal origin within the combustion system. These mercaptans, dialkylated sulphides and disulphides (e.g. include compounds such as benzaldehyde and dimethyl sulphide and hydrogen sulphide). These nitromethane observed in the exhaust emissions from species oxidise during combustion to sulphur dioxide landfill gas fired internal combustion engines and (SO2) and, to a much lesser extent, sulphur trioxide turbines, but not in the landfill gas fuelling such plant. (SO3) – collectively known as oxides of sulphur (SOx). Typical emissions recorded from a range of landfill gas These compounds, in turn, may react with water flares are given in Appendix D. Complete combustion vapour at lower temperatures to yield sulphuric acid. of landfill gas will reduce the impact of emissions Hydrogen chloride (HCl) and hydrogen fluoride whether related to health, environment and amenity. (HF). These acid gases are produced during the The potential impacts of raw landfill gas and emissions complete combustion of landfill gas from the wide from combustion systems are compared in Table 6.8. range of chlorinated and fluorinated organic compounds that can be found in the gas. These highly Table 6.8 Comparison of potential impacts of reactive acid gases are associated with accelerated rates raw landfill gas and emissions from of corrosion of plant and equipment in landfill gas combustion systems utilisation schemes. Raw Incomplete Complete Particulates. The majority of landfill gas utilisation Impact landfill combustion combustion systems have filters designed to remove particulate gas of landfill of landfill matter above 0.3 to 10 mm in size. This is done to gas gas protect the system from adverse rates of abrasion, although some US research suggests that even solids Explosion risk ✓✓✓ ✓✓ 0 with a diameter of less than 1 mm can result in Toxicity and increased wear. Although a significant fraction of the ✓✓ ✓✓ ✓ asphyxia particulate burden in the gas will be removed before combustion, additional particulate matter may be Odour ✓✓✓ ✓✓ ✓ produced during the utilisation processes and from the Phytotoxicity ✓✓ ✓ plant post-filtration. This matter is likely to include metal Stratospheric salts derived from the corrosion of plant and ✓✓ ✓ ✓ ozone depletion equipment, and carbon produced by incomplete combustion. Global warming ✓✓✓ ✓✓ ✓ Photochemical Polychlorinated dibenzodioxins (PCDDs) and ✓✓✓to ✓✓✓ smog polychorinated dibenzofurans (PCDFs). These are thermal products arising from the combustion of Acid gas formation 0 ✓✓ ✓✓✓ materials/fuel containing both organic compounds and 0 = No impact chlorinated compounds. Their formation is favoured ✓ = Comparatively low potential impact where the emissions contain particulates (which provide ✓✓ = Comparatively moderate potential impact a reaction surface for the formation reactions). PCDDs ✓✓✓ = Comparatively high potential impact and PCDFs have typically been found at relatively low concentrations in the emissions from landfill gas combustion plant. Non-methane volatile organic compounds (NMVOCs). A wide range of organic compounds have reportedly been detected in both landfill gas and the emissions from a variety of landfill gas combustion plant at trace concentrations. The compounds detected in
68 Environment Agency Guidance on the management of landfill gas 7
Gas control measures
The following chapter sets out specific measures and Designs should be set out using drawings and techniques for controlling landfill gas, based on the specifications supported by calculations and method regulatory requirements identified in Chapter 4. statements. 7.1.2 Construction quality assurance 7.1 Design and construction quality assurance Construction quality assurance (CQA) refers to the means and actions employed to assure conformity of 7.1.1 Design all the elements of the gas control measures, The design of each element of the gas control system including installation in accordance with the drawings needs careful consideration in the context of its end and specifications. It forms an essential part of site use and site setting. All elements of the proposed development. The CQA measures applied should be control systems should be designed and assessed in pragmatic and commensurate with the potential accordance with recognised standards and consequences of failure of the plant and equipment methodologies. These processes should be concerned. documented to provide an adequate audit trail. The CQA plan should cover: The approach to the design process should be ● responsibilities discussed with the regulator. Designs should reflect ● specification of products and materials the development of the conceptual model. ● handling and installation processes The design of gas control systems should consider ● testing and inspection the: ● requirements for plant commissioning and trials ● requirements for validation process. ● performance required to achieve the standards derived from the risk assessment; A CQA plan must be prepared for all elements of the ● context of the elements of the gas control system control systems (containment, collection and under consideration, e.g. whether a temporary or treatment). Following completion of each phase of a permanent system; the installation works, a CQA validation report must ● design life of the elements of the gas control be prepared. The report should provide details that system; the works have been carried out in accordance with ● purpose of the elements of the gas control system the approved designs and CQA plan. It should and the environment in which they are situated; include: ● selection of materials and products; ● details of the nature and extent of works ● compatibility of the installed elements of the undertaken control system in terms of the phased ● CQA records development of the site, e.g. the sizing of ● conformance test results appropriate gas extraction plant for the gas ● details of relevant commissioning and trials production of the site and not just individual ● as constructed drawings of the works. phases or power output; ● operational and maintenance requirements; The CQA plan should encompass works set out in the ● health and safety issues. site emergency plan and undertaken as emergency measures. Designs should be prepared to a sufficient level of detail to allow them to be easily interpreted and The installation of site containment (lining and implemented. Where possible, construction should be capping) is likely to be undertaken separately from carried out using commonly available techniques. that for gas collection and treatment systems. Where this is not possible, sufficient details should be However, this will not be the case with built-up provided to allow alternatives approaches to be collection wells installed during site lining and the considered.
Environment Agency Guidance on the management of landfill gas 69 interface between collection and monitoring wells installation of the gas collection and control system. with the site cap. Depending on the nature of the The following elements should be evaluated as part of works, installation should be supervised either by in- the planned development of the site: house personnel or a third party. ● engineering of contained void space to meet the The minimum requirements for the validation of needs of waste placement at the site; collection and treatment systems in order to ● ensuring measures for gas collection and demonstrate the integrity of the installed measures at treatment are implemented to meet the needs of the time of installation (i.e. not to demonstrate on the anticipated rate of development and gas going performance) are as follows: production at the site; ● visual inspection of pipework prior to covering; ● planning the capping of individual phases to ● functional testing of the integrity of pipework (e.g. provide the earliest practical gas containment to pressure testing) to an appropriate standard (and meet the needs for gas control. commensurate with the function of the pipework Other constraints such as odour (e.g. collection of as required by the design) to verify competence of landfill gas from operational phases to deal with the pipework and joints; odour problems), noise and hours of operation also ● supervising and recording the installation of require consideration at all stages of site preparation, collection wells; operation and development. ● surveying the location of pipework, collection wells and other installed control measures; 7.2 Containment ● establishing that collection pipework and wells have been constructed in accordance with the 7.2.1 Lining design, e.g. verifying the correct pipe The basic requirements for landfill lining are set out in diameters/well size, well depth, pipe gradients and Section 4.2.2. For new sites, they arise from Annex I locations; of the Landfill Directive and Schedule 2.3(2) of the ● checking that all elements of the collection Landfill Regulations as follows: systems and treatment plant meet the design and the objectives set out in the CQA plan; ● details of plant commissioning and trials. Soil, groundwater and surface water is to be protected by the use of a geological barrier combined with: The preparation of the designs, CQA plan, supervision (a) a bottom liner during the operational phase/active of installation works and the preparation of the CQA phase of the landfill; validation report should be undertaken by a competent person in consultation with the regulator. (b) a top liner following closure and during the aftercare phase 7.1.3 Phasing
During the planning of site development, All landfills (new and existing) must meet the consideration should be given to the phasing of: following fundamental requirements: ● landfill lining ● a low risk of trigger levels or emission standards ● waste placement being breached over the whole lifetime of the ● implementation of gas controls (collection and landfill; treatment systems) ● landfill engineering must have structural/physical ● capping/restoration. stability in the short-, medium- and long-term. Site phasing should be part of the conceptual model. A variety of natural and artificial lining materials are This will allow critical stages of site operation to be commonly available, although the use of natural identified and appropriate control measures to be materials normally depends upon their availability designed and set out in the Gas Management Plan. near the site. Lining materials include: The Gas Management Plan must clearly define the sequence of development, operation and ● engineered clay capping/restoration. ● bentonite enhanced soils (BESs) ● geosynthetic clay liners (GCLs) The emphasis for the control of landfill gas should be ● geomembranes: to optimise the use of engineered containment in – high density polyethylene membranes (HDPE) association with gas collection/control. Following (typically used) waste placement, particular attention should be given – medium density polyethylene membranes to the programming of site capping and the (MDPE)
70 Environment Agency Guidance on the management of landfill gas – linear low density polyethylene membranes If the competent authority after a consideration of (LLDPE) the potential hazards to the environment finds – polypropylene (PP); that the prevention of leachate formation is ● dense asphaltic concrete (DAC). necessary, a surface sealing may be prescribed. The lining systems necessary to fulfil the requirements Recommendations include a topsoil cover >1 metre at of the Landfill Directive can consist of a composite of non-hazardous and hazardous sites. Provision is to be different types of these materials. The use of these based on a site-specific assessment. A subsoil/top soil lining materials may involve the incorporation of an layer above the capping layer to provide protection artificially enhanced attenuation layer such as clay or will prevent desiccation cracking and can also be a BES layer with a geomembrane liner such as HDPE. beneficial when the materials permit the oxidation of The lining system should be designed to take account methane (see Section 7.4.5 for details of this process). of the development of the conceptual model in Geomembranes may also require additional relation to the gas permeabilities of the individual protection to prevent puncture, particularly during components. the placement of the restoration soils. Such The selection of lining types will be driven by the protection measures may involve the incorporation of capacity of the lining system to provide groundwater geotextiles, geo-nets or sand layers, which can also protection as well the containment of landfill gas. function as a drainage layer above the cap. Careful consideration should be given during the Preventing point source emissions at the interfaces development of the conceptual model to the choice with the containment system around the perimeter of of the elements of the containment system. the site and at engineered features in the cap (e.g. for Potential failures in landfill gas control due to seals leachate and gas monitoring and collection wells) between the side wall liners and capping being should be considered carefully at the design stage. disrupted by settlement should be specifically Appropriate methods of monitoring and maintaining considered in the design of appropriate monitoring of the performance of these features, following lining systems. Lining systems should be designed installation, should be incorporated (see Chapter 8). and installed in accordance with the criteria set out in Risk assessment and the development of the Section 4.2.2. conceptual model should be used as a tool for the 7.2.2 Capping design of the gas containment and collection systems. The capping of unlined landfills may Common types of capping materials used include: encourage gas migration laterally, and special care ● low permeability mineral layers: must be taken when designing the gas management – engineered clay system for such sites in order that gas can be – bentonite enhanced sand; successfully collected from the entire depth of the ● artificially sealing layer (geomembranes): landfill. The type of the collection system and the – HDPE rationale for operation, utilisation and flaring may be – LLDPE considerably different from that for a fully contained – GCL. site, where the focus on gas migration may not be as significant. Emission standards (applicable in England and Wales) have been established for permanently and Like lining systems, capping layers should be subject temporarily capped landfills (Environment Agency, to the same process of design and CQA as described 2004d). These should be considered during both the in Section 7.1.2. risk assessment and the preparation of the gas control measures including capping design (which will lead to the selection of the capping material). Particular attention should be given at the design stage to the mechanisms by which changes in the waste mass (e.g. settlement) may affect the cap’s integrity. Regardless of whether there is an active gas collection system, surface methane emissions can occur through imperfections in capping systems due to the constraints of construction and the limitations of the materials. The Landfill Directive provides recommendations for capping:
Environment Agency Guidance on the management of landfill gas 71 7.3 Gas collection deteriorate due to general damage, settlement, biofouling and leachate perching. The regulatory requirements for the control of landfill gas are set out in Chapter 4. Before the design and/or If it is not possible to install a permanent collection installation of permanent gas collection systems, an system, the use of temporary or sacrificial wells appropriate risk assessment is required to should be considered in order to control landfill gas demonstrate the suitability of the proposed controls. during the operation of the site. Shallow gas probes or ‘pin’ wells (see Figure 7.2) can be used to give The following section provides details on the methods additional or temporary controls in such and techniques for gas control. The detailed design of circumstances and are often used to provide pipe networks can be complex and expert assistance protection against uncontrolled gas movement in is likely to be required to complete the design of this areas such as sensitive boundaries prior to capping. part of the Gas Management Plan. However, the effectiveness of such shallow The layout of the gas collection system should be probes/wells should be reviewed regularly. Such designed after careful consideration of: collection measures are also not appropriate for the recovery of landfill gas at depth. The installation of ● risk screening and the development of the shallow probes and pin wells should also be subject conceptual model; to appropriate CQA measures as set out in the CQA ● positions of gas collection wells; plan. ● preferred alignment of permanent connection pipework in relation to the pre- and post- Collection wells are normally formed by the settlement profiles, and temporary control incorporation of perforated pipework, surrounded by pipework during operation of the site; a natural stone or crush aggregate with a low ● the design and sealing of leachate collection calcareous content. Specifications vary, but the systems (particularly how they may affect the materials and products should be suitable for the efficiency of gas collection by providing routes for application in which they are to be used (Cooper et air ingress); al., 1993). ● consideration of the need to provide condensate drainage; ● the need to make provision for expansion and contraction of the collection pipework; ● proposed capping and restoration; ● phasing of site operations; ● facilities for routine maintenance and system disconnection. Guidance on the design and installation of gas collection systems is given in Section 7.1. Gas pumping trials are often undertaken to establish the performance of the gas collection system, and to assist decisions in the design process and the selection of extraction plant and other equipment. 7.3.1 Collection wells Networks of landfill gas collection wells are normally installed to enable the removal of landfill gas from the waste in order to achieve the objectives of the Gas Management Plan. Gas wells can either be built up as landfilling proceeds or bored into the waste mass following filling. A variety of designs is available depending upon site- specific issues such as the depth of waste or the rate of infill (see Figures 7.1 and 7.2). Some use has also been made of horizontal collection wells installed within the waste mass in trenches. The performance of both horizontal and vertical wells is known to Figure 7.1 Well arrangements for a variety of different landfill site types
72 Environment Agency Guidance on the management of landfill gas Layout the gas from the site at a number of levels. Schematics for some collection well arrangements are The diameter and spacing of collection wells can vary shown in Figure 7.1. and will depend on a number of site-specific factors. The layout should therefore be designed using a risk- The configuration of the gas collection wells must based approach. Although there are no absolute rules consider the requirements for perimeter collection in for the spacing of gas wells, it should be typically no relation to lateral migration. This should be balanced more than 40 metres. Closer spacing of collections with the need to control surface emissions and wells may be necessary to improve control in recover landfill gas from the lower parts of deep particularly sensitive areas; this will vary depending landfills. on site-specific conditions, the effective zone from Where deemed appropriate, the design and layout which the well will draw gas and the operating should incorporate the use of a gas drainage layer. In protocol for the site. The spacing is also determined addition, the design must ensure that migration by the depth of waste; for instance, in deep sites, control is not compromised by the utilisation of nests of wells can be installed at different depths to landfill gas. Alternative or separate systems may be facilitate the effective collection and extraction of gas. required to overcome this problem. The configuration of the wells and the depth of the Careful consideration must be given to the location of site should be considered carefully at the design stage collection wells in relation to the proximity of other to ensure that gas can be extracted effectively from engineered features and the requirements for the the waste mass. The depth, spacing and layout will placement and compaction of waste in these areas. be dictated by the internal geometry of the site, The need to provide measures for the balancing and particularly in land raise or deeper landfills. In such maintenance of collection wells is another important instances, it may be necessary to install wells to a issue. range of depths to provide facilities for collection of
Key 1. Head chamber set in capping soils 2. Concrete base 3. Bentonite seal 4. Joint 5. Unperforated casing 6. Perforated casing 7. Fines-free crushing stone 8. Crushed stone filled ‘gabion’ or similar 9. Containment system 10. Base detail 11. Cap
Figure 7.2 Examples of collection well arrangements
Environment Agency Guidance on the management of landfill gas 73 Design of wells protected against accidental damage and vandalism. Alternative arrangements for the connections within the General arrangements for examples of gas well types well head chamber and a number of proprietary are shown in Figure 7.2. Careful consideration should systems are also available which provide adequate be given to the selection of well types in relation to the monitoring and control of the gas well. An example of well’s function and the collection strategy. This will a well head arrangement is given in Figure 7.3. necessitate designing the collection system and layout for site-specific conditions such as site geometry, Constructing collection wells vertically or horizontally operational phasing, waste characteristics etc. – which during landfilling allows gas to be collected before a may vary throughout the operation of the site. particular phase is completed, thus providing greater flexibility in the control of landfill gas at an earlier stage The head of the well, which provides the point of in the site’s development. connection to the gas collection pipework and access for monitoring and maintenance, can be constructed When designing built up wells, consideration should be using different arrangements depending on the afteruse given to the instability of the well as a result of waste and/or requirements for access. Typical monitoring placement and settlement, which may lead to a facilities at the well head allow for gas sampling, flow deterioration in performance. Proprietary built-up well rate/pressure measurement and dipping of the well. systems are available in a variety of diameters and can provide an effective solution to settlement difficulties. Control valves should be incorporated to allow automated or manual adjustments to each well. This Consideration should also be given to the base detail of can be achieved either at the well or, in the case of collection wells, where the interface with the manifold systems, through the connection of several containment system can potentially cause damage. wells to a control chamber. Retrofitting collection wells by drilling from the surface The well head chamber is often incorporated within the of the site following waste placement can present a restoration layers (i.e. the well head is below the number of potential difficulties in terms of physically restored ground surface) because of afteruse and/or installing the well due to obstructions in the waste, the visual requirements. likelihood that point source emissions will be generated during installation, and the need to provide temporary At some sites and especially those that have not ceased control prior to installation. landfilling, the well head can be built above ground level. This has the advantage of providing simple access Care is also required during the retrofitting of collection for adjustment and monitoring, although it must be wells to ensure that the containment system is not compromised. A distance of at least 1–2 metres should be maintained between the base of the well installation and the containment system. Retrofitted wells should be installed in accordance with code of practice published by the British Drilling Association (2002). The design detail of the interface between the cap and well should be considered carefully as the integrity of the cap can often be compromised in areas where differential settlement and failure of the cap occurs. Regular inspection for settlement in these areas should be undertaken as part of routine site operations.
Figure 7.3 An example of a well head arrangement
74 Environment Agency Guidance on the management of landfill gas Operation of wells compatibility with other elements of the gas control measures in relation to: In general, gas collection wells should not be used for leachate recirculation due to local saturation of the ● how gas will be collected and removed from the waste, which can lead to a reduction in the effective layer; (i.e. unsaturated) length of the perforated casing of the ● interface with the cap (protection and well. deterioration); ● the potential to draw air into the collection The potential deterioration in gas well performance is system; particularly important. This can arise for a number of ● interface and compatibility with other features of reasons, including: the landfill gas collection system; ● reduced gas conductance of the waste and stone ● settlement of the waste; fill surrounding the perforated casing caused by ● the potential drainage of perched leachate silting (i.e. the pores become progressively through the drainage layer; obscured with fines); ● access for monitoring. ● blockage of the perforations in the casing by 7.3.3 Collection pipework sediments and bacterial growth; ● overdrawing of gas wells by inducing excessively Connection pipes link the gas collection wells with the high flow rates; treatment system. They provide a controllable means ● mechanical disruption of the gas well by ground for gas extraction, thus promoting acceptable and movement or settlement causing a reduction in reliable gas management. This is normally achieved the cross-sectional area available for gas flow. using various sizes of collection pipes in either HDPE, MDPE or PP. Manufactured lengths of pipes are It is important to ensure that adequate monitoring and connected together to form runs to suit the site inspection of the wells is undertaken to detect if any of geometry. Connections can be made using a number of these problems have occurred. In addition, gas well techniques (including butt fusion welding or the use of design should make allowance for these various modes either electro-fusion or flexible band seal couplings), of failure. This can be achieved by adopting a depending on the function of the pipework, e.g. pragmatic design based on the following criteria: temporary or permanent. ● maximising the effective length of the gas well, The layout design should allow for: i.e. by controlling leachate accumulation within the waste mass possibly by extraction from the ● gas monitoring gas well and, in the case of retrofitted wells, by ● prevention of blockage and disruption (by water, drilling as deep as practicable without leachate or condensates and waste movement) compromising the basal containment system; ● routine operations and maintenance tasks. ● providing contingency measures to maintain Provision for the de-watering of pipelines and plant and consistent operation (e.g. built-in slip-joints or the management of landfill gas condensate should be flexible connections at the well head) while considered as part the design of the control system. allowing for the effects of site settlement on the This should be addressed within the Gas Management gas well. Plan. 7.3.2 Collection layers Undisturbed ground (no settlement) is the preferred Drainage layers, which can form an integral part of the location for major or principal pipe runs. During the gas control measures, assist the collection of landfill gas operational phase, however, collection pipework is often through the incorporation of: laid temporarily over the filled surface prior to the installation of permanent collection systems, which are ● aggregate layers (sand, gravel, crushed stone); normally laid in trenches in the site restoration soils ● geocomposite layers such as geo-nets. above the capping layer. Measures to protect these However, the guidance on capping requirements given temporary arrangements from damage should be in Landfill Regulatory Guidance Note 6 (Environment incorporated. These may include physical protection Agency, 2003c) states that the Landfill Directive such as cones or concrete blocks, and markers such as recommendations for capping can be changed on the warning tape or signs. Surface laid pipelines may be basis of a risk assessment. Consequently, a gas drainage exposed to a higher potential for damage arising from layer may not be required at non-hazardous landfills. high winds and expansion/contraction induced by ambient temperatures. This is likely to necessitate more When considering the incorporation of the gas frequent integrity testing and inspection. collection layer, it is necessary to examine its
Environment Agency Guidance on the management of landfill gas 75 The key factors determining pressure distribution include: system, which must be confirmed before commissioning by means of pressure testing as part of ● surface roughness the CQA requirements. ● gas density ● length Options for connection patterns for gas well fields ● cross-sectional area and well arrangements are shown in Figure 7.4. The ● flow velocity (i.e. the flow rate, in m3/s, divided most appropriate layout for a particular site will by the cross-sectional area of the pipe in m2). depend upon a range of factors including: Compressors and boosters are discussed in Section ● the availability of undisturbed ground 7.3.4. Practical experience with existing systems has ● the geometry of the site shown that for ‘long legs’ (i.e. greater than about 5 ● the restoration profile and local topography metres), the flow velocity should not exceed about 10 ● environmental constraints that may dictate the m/s. This ‘rule of thumb’ results in the selection of location of the processing and treatment plant, 50–100 mm nominal bore (NB) for spurs connecting to e.g. visual intrusion. well heads, up to 300 mm NB for headers linking The number of variables means that there will be no multiple wells and up to 450 mm NB for larger headers single solution to the collection layout. Therefore, the and ring mains. The operational effectiveness of the approach should be to identify several layouts, which connection pipework is related to the integrity of the
1. Ring Main System
Treatment and Flaring
2. Multiple Header System
Treatment and Flaring
3. Single Main with Outfield Regulation
Treatment and Flaring
Key: Collection Wells
Gas Extraction Plant
Manifold
Control Valve
Collection Pipework
Perimeter of Landfill
Figure 7.4 An example of landfill gas collection system options
76 Environment Agency Guidance on the management of landfill gas should be reviewed in consultation with site ● connection arrangements operational engineers before making the final choice. ● viscosity of the gas mixture. Detailing of the pipe runs in terms of pipe size The design strategy should be to select values for selection and trench depth can then begin. At this suction and delivery pressures from a given point, the requirement to minimise accumulation of compressor or booster type within their range of water (condensate) in the pipework should be performance characteristics, and then specify addressed by incorporating de-watering features such pipework for the required landfill gas flow rate. as gravity drains into the site or gas wells. Alternatives include in-line knockout vessels and de-watering A variety of compressors or boosters with individual manifolds, both of which may either be gravity- uses are available in a range of capacities, allowing drained or pump-discharged. selection to suit the site’s needs. Table 7.1 gives details of some commonly available compressors and Consideration must also be given to the well head boosters. type and arrangement. In terms of appearance and security (particularly on closed sites), the preference is Centrifugal and regenerative machines are commonly for below ground designs. However, it will be used as they are well suited to typical requirements necessary to provide access at key points for for landfill gas extraction. Incorporating other monitoring, maintenance and adjustment. processes prior to utilisation or flaring (e.g. deliquescent dryers or filters) introduces additional 7.3.4 Extraction plant pressures, thus increasing the requirement for suction Gas must be drawn through the collection pipework capacity. In this case, it may be appropriate to use to the point of treatment. This is normally achieved higher pressure rated machines such as roots blowers by incorporating compressors or boosters to generate and, less frequently, reciprocating compressors. a pressure differential, which are capable of Care should be taken when selecting both the overcoming the total pressure loss from the gas wells. extraction equipment and the type of collection Pressures losses that occur up to the inlet of the control systems to prevent over extraction as this can compressor are known as ‘suction’ losses and those draw air in to the waste and present a fire hazard. from the exit of the compressor to the point of Fires can also be caused by damaged or poorly treatment as ‘delivery’ losses. The magnitude of the functioning gas collection systems that permit air to suction and delivery losses is a function of the: be drawn into the waste. The gas collection system and the cap should be well engineered and ● gas flow rate maintained to prevent fires. ● length and internal diameter of the pipes ● smoothness of the pipes
Table 7.1 Examples of types of compressors and boosters
Type Typical flow Typical pressure Comments rate (Nm3/hour) rise (mbarg)
Single-stage 2,000 130 Well suited to landfill gas extraction and by far the centrifugal booster most common machine used. Low maintenance costs and tolerant of ‘dirty’ gases. Two-stage centrifugal 2,000 200 Well suited to landfill gas extraction and supply to booster consumer. Frequently used to supply electricity generating sets. Regenerative booster 1,000 200 Suitable for landfill gas, although much less frequently used than centrifugal boosters. Roots blower 1,000 1,500 Occasionally used for landfill gas to supply generators. Positive displacement machine that does not tolerate liquid water. Sliding vane 1,000 1,000 Similar to Roots machines. Relatively high rotary compressor operating and maintenance costs. Reciprocating 1,000 >50,000 Capable of very high supply pressures. Have been compressor used to feed gas processing systems. High operating and maintenance costs.
Environment Agency Guidance on the management of landfill gas 77 Regular monitoring of the gas collection system will The basic approach to condensate management also facilitate the prevention and early detection of focuses on eliminating liquid directly from the gas fires, enable balancing of the gas field and ensure collection pipework using a combination of: that landfill gas is being extracted efficiently with due ● well head de-watering regard for migration. ● low point drainage through water-sealed traps 7.3.5 Condensate management ● collection and disposal at either a knock-out vessel or at one of a series of drained manifolds/points. The conditions which lead to the formation of landfill gas give rise to a gas mixture at a temperature Details of a typical condensate drainage point are typically in the range 30oC to 40oC, with a relative shown in Figure 7.5. saturation at or approaching 100 per cent. When the Effective design relies on setting pipe runs to fall gas stream cools, water vapour condenses to form towards drainage points typically with a minimum ‘condensate’ which can accumulate in collection gradient of 2 per cent. If such falls cannot be pipework. In addition, actively drawing landfill gas achieved due to the constraints of either the from a gas well can, under certain conditions, give restoration profiles or available land, then the rise to liquid entrainment, usually in the form of a pipework can be stepped to give a ‘saw-tooth’ froth or foam. alignment. In this case, a ‘drop-leg’ is incorporated at Liquid condensate or leachate in gas pipelines appropriate intervals along the pipework from which reduces their effectiveness and can even lead to water can be drained. complete blockages and major disruptions. Therefore, A typical arrangement for incorporating condensate measures should be incorporated into the gas drainage in gas collection pipework is shown in management system to reduce and control liquid Figure 7.6. accumulations.
Figure 7.5 Typical condensate drainage point
78 Environment Agency Guidance on the management of landfill gas Figure 7.6 Typical condensate drainage collection pipework
Note: water traps may be equipped with pumps for condensate difficulties in terms of system performance/failure, collection where soakaways are not used e.g. the deterioration of valves and other plant, and/or the leakage/loss of lubricating oils into the gas If the condensate is removed on-site, then it can be stream due to the failure of seals. The properties of recirculated within the waste (if this is allowed). If the the condensate should also be considered in terms of condensate is taken off-site for disposal as a health and safety, particularly in relation to dermal controlled waste, it should be handled and managed contact during maintenance. according to its properties. Typical properties of landfill gas condensate are given Due to its potentially corrosive nature, the properties in Table 7.2. of the condensate should also be considered when designing and specifying elements of the control and treatment system. This presents a number of
Table 7.2 Typical characteristics of landfill gas condensates*
Plant/flare Gas field drains
Parameter Typical Typical Typical Typical upper value lower value upper value lower value
pH 7.6 4.0 3.9 3.1 Conductivity 5,700 76 340 200 Chloride 73 1 4 <1 Ammoniacal nitrogen 850 <1 15 3 TOC 4,400 222 9,300 720 Chemical oxgen demand (COD) 14,000 804 4,600 4,600 Biochemical oxygen demand (BOD) 8,800 446 2,900 2,900 Phenols 33 3 17 4 Total volatile acids 4,021 141 4,,,360 730 * All values in mg/litre except pH (dimensionless) and conductivity (µS/cm)
Adapted from Knox (1990).
Environment Agency Guidance on the management of landfill gas 79 7.4 Utilisation, flaring and treatment 7.4.2 Process control Additional levels of primary treatment/supplementary The aim of a process control system is to ensure that processing should be introduced when the gas is to the design ratings and parameters of the control be used as a fuel. These can include: system are adequately achieved and maintained. In the context of primary processing, the key variable is ● filtration the flow rate of landfill gas, which in turn is governed ● drying (or ‘conditioning’) by the values of the suction and delivery losses. The ● higher pressure boosting principal control variable is pressure, for which there is ● after-cooling a range of measurement equipment options. ● gas composition adjustment. Pressure control can be achieved using several Another consideration is treatment to remove different valve types and may be either manual or potentially corrosive trace contaminants such as automatic (balancing). For small and relatively simple organochlorides, organofluorides and hydrogen gas management systems, manual control can be sulphide (see Environment Agency, 2004c). adopted. Automatic pressure control relies on a Utilisation and flaring involves the combustion of spring-controlled diaphragm to adjust the position of landfill gas – with recovery of the energy content, the valve seat in response to continuous sensing of the where appropriate – to form an off-gas, which is supply pressure. acceptable for direct discharge to atmosphere. At The most commonly used valves are of the butterfly sites producing landfill gas where it is not possible to type, although the use of ball valves and gate valves generate energy, the minimum requirement is flaring may also be appropriate. – sometimes with the use of time switches to deliver intermittent flaring. A key requirement for any control valve in a landfill gas management system is an appropriate level of Treatment of the gas stream either pre- and/or post- corrosion protection. In practice, this has led to the combustion may be required to meet the emission use of an all-plastic (polyethylene) construction or limits for the processes identified above. This will be a stainless steel/aluminium–bronze discs and spindles site-specific issue based on the precise composition of and polymer-coated bodies. the gas stream and will not constitute the norm. The use of automated control reduces operational Useful information on the overall approach to landfill input and these systems when linked, via telemetry, gas processing and treatment is given in a report can provide an effective means of systems monitoring. produced by ETSU for Dti (ETSU, 1996). There is, however, a need to ensure appropriate levels 7.4.1 Gas treatment and supplementary of regular monitoring, inspection and maintenance processing when such systems are used and these should be clearly set out in the Gas Management Plan. Larger or more complex landfill gas management systems, and especially those supplying gas to a 7.4.3 Utilisation consumer, require additional treatment to provide Key factors that should be considered during the greater control of the condition of the delivered gas. design of the utilisation plant include: There is no single standard system and the nature of the treatment will generally be defined in the form of ● composition of the raw gas extracted/used from a specification covering the following issues: the landfill; ● level and type of pretreatment or conditioning ● a minimum supply pressure; applied to the gas prior to its supply to the ● the target calorific value or Wobbe index (see combustion equipment (e.g. water removal and Glossary); filtration); ● the supply temperature, dew point or specific ● type of combustion equipment used (e.g. internal moisture content; combustion engines with wet or dry manifolds, ● entrained particle size limits; gas turbines, etc.); ● gas composition restrictions (e.g. proportions of ● temperature of combustion; corrosive gases such as hydrogen sulphide); ● set-up and maintenance of the combustion ● combustion characteristics such as oxygen content equipment; and flame speed. ● fuel to air ratio applied during combustion (which Further details on gas treatment and supplementary will affect the amount of excess air, if any, processing are given in Environment Agency (2004c). available and hence the completeness of oxidation reactions);
80 Environment Agency Guidance on the management of landfill gas ● use of secondary or quenched air; Table 7.3 Typical upper limits for some trace ● potential post-combustion treatment of emissions compounds in landfill gas utilisation and recirculation of exhaust. plants Although direct use of the thermal energy is Parameter/component Typical upper technically relatively straightforward, there are few limit situations where a consumer is located close enough 3 to the source of the landfill gas to justify cost-effective Sulphur dioxide 0.1 mg/m supply. Utilisation schemes in which the thermal Hydrogen sulphide 25 mg/m3 energy is used to generate electricity, which is then Total organosulphur compounds 150 mg/m3 exported, are much more common. Another option 3 for utilisation is the clean-up and storage of landfill Total organofluorine compounds 25 mg/m gas for use as a vehicle (or mobile plant) fuel. Other Total organochlorine compounds 60 mg/m3 applications for landfill gas utilisation are given in o Supply temperature T + 5 C Table 7.4. sat Particles 7 µm Utilisation of landfill gas for electricity generation has proved popular owing to the incentive provided by Note: Tsat = temperature at saturation the Non Fossil Fuel Obligation (NFFO) and, more Treatment options for reducing the concentration of recently, by the policy on new and renewable energy corrosive components in landfill gas are available. The as laid down by the Renewables Obligation (Dti, most common approach taken by operators is to 2000) and the Renewables Obligation (Scotland) implement a carefully formulated monitoring protocol Order (Scottish Executive, 2002b). and relate the findings to planned maintenance of the The most common technology employed in the UK engine. For example, monitoring and frequent for electricity generation is the spark ignition engine. changing of lubricating oil can be adopted to overcome This type of engine accounts for over 86 per cent of the effects of corrosive volatile compounds (Fisher, recent landfill gas to energy schemes in the UK. 1992). The processing and treatment of landfill gas intended Further guidance on the technologies associated with to fuel electricity generators is broadly the same as for the clean-up of landfill gas are given in Environment other thermal utilisation processes. However, the Agency (2004c). Consideration should be given to the specification for moisture content may be tighter, location of utilisation plant; the impact of exhaust gases requiring careful design of gas conditioning and needs to be evaluated through risk assessment before prevention of downstream cooling (which could give finalising its design and location. rise to condensation). This requirement is especially 7.4.4 Flaring relevant to turbocharged spark ignition engines fitted with fuel–air control devices. The final stage of processing in the absence of utilisation is thermal oxidation of the landfill gas in a A further requirement may be the control of flare. Flares are also used as a standby process to treat potentially corrosive compounds or components, gas in periods of utilisation downtime. which can have an adverse effect on lubricating oil characteristics. Hydrogen sulphide, organofluorides The capacity of the flare system must be compatible and organochlorides all fall into this category. The with the operational parameters of the site over time. acceptable limits for concentrations of such This is particularly important where flares are used in components have generally been determined through conjunction with utilisation as a standby or control for operating experience. Engine manufacturers excess gas. The operating rationale should be clear and frequently list values in fuel specifications for their risk-based in relation to such scenarios. particular machines and offer warranties based on Historically, open diffusion type flares have been used to defined gas compositions. This issue should be given flare landfill gas. These types of flares are no longer careful consideration when selecting the plant and acceptable as they do not ensure consistent combustion equipment, as it may give rise to operational and cannot be monitored. More sophisticated designs limitations and should be established during gas must be adopted in order to achieve permanent production trials. Typical upper limits of selected trace control, enhance thermal destruction efficiency and compounds in gas utilisation plant are given in minimise the formation of secondary pollutants. Table 7.3. Technical guidance on landfill gas flaring has been produced by the Agency (Environment Agency, 2002d), which sets out the requirements for flare selection and
Environment Agency Guidance on the management of landfill gas 81 design. Monitoring methods and emission standards ● ability of oxygen to permeate soil to reach site of can be found in other Agency guidance (Environment oxidation, i.e. the permeability of the cover soils Agency, 2004b). and host lithology; ● thickness and condition of the cover soils above Maximising the thermal destruction efficiency and any engineered cap (or waste if there is no minimising the yield of NOx can be achieved by careful engineered capping layer); control of combustion air and imparting a high level of ● rate of landfill gas flux through the soil; turbulence upstream of the flame front. Flares are design of the lateral and capping liners, and the manufactured using a range of proprietary designs with types and distribution of defects in those lining varying capacities and with appropriate monitoring systems; facilities. The use of low calorific thermal oxidation ● presence or absence of cracks in the cover soils, or systems provides a method of providing gas control fractures in the host lithology; during the early and the later stages of gas production ● biochemical factors such as nutrient availability, at landfill sites with as little as about 1 per cent soil pH, temperature and moisture content. methane. Methane oxidation will have a role in the management of Other design elements that should be considered landfill gas control where the following conditions exist: include the height of the stack, the requirement for appropriate retention times and the incorporation of ● the landfill is toward the end of its gassing life and appropriate sampling ports. Current practice (often active gas collection is unable to collect a gas of governed by planning constraints) is normally based on sufficient quality for flaring, even using low minimising the visual intrusion of the unit without calorific thermal oxidation systems; sufficient consideration of the plume dispersion ● soil cover is well aerated; characteristics or noise. ● soil cover can be inspected regularly to ensure cracks are absent and the soil structure is These are key matters that must be addressed and maintained to a good standard. further guidance on noise is provided by IPPC Horizontal Guidance Note H3 (Environment Agency, Biological methane oxidation as the sole management 2002g). The optimal stack height will depend on the: tool for controlling methane emissions is, therefore, only potentially suitable for those sites which are ● exit velocity significantly beyond the peak of their gassing life or ● pollutant loading where other active forms of gas control will not perform ● retention time required (typically 0.3 seconds at o effectively. However, the benefits of methane oxidation 1,000 C) should not be ignored at any site. ● location of the flare in relation to receptors ● surrounding topography. 7.4.6 Other mechanisms for treatment and utilisation Dispersion modelling should be performed before finalising the design and location of the flare. Although uses of landfill gas are generally limited to electricity and heat generation there are others. Forbes 7.4.5 Methane oxidation et al. (1996) describe two processes used in the USA for Microbial populations in soils can oxidise a proportion the clean-up of landfill gas to yield methane for use as a of the methane passed through them. This has been vehicle fuel. One process produces a compressed form shown in both field and laboratory studies (DoE, 1991b; of methane and the other produces liquid methane. 1991c). The Agency is undertaking research into the These novel uses, together with other uses are role and practicalities of methane oxidation at landfill summarised in Tables 7.4 and 7.5. sites (Environment Agency, 2002h). 7.4.7 Support fuels The percentage of methane oxidised depends very It may be necessary to use a support fuel that can be much on the residence time of landfill gas in the soils injected either intermittently (on demand) or and the volume of landfill gas fluxing through the continuously into the flare in situations where: surface. Availability of oxygen to the site of oxidation is the rate-limiting step for the process. ● it is critical to maintain extraction, collection and flaring of landfill gas; The microbial population responsible for methane ● the gas composition is insufficient to sustain oxidation can develop in the capping/restoration layers efficient combustion. or in the host lithology and soil surrounding the site. The factors that can influence the extent of microbial In such cases, the use of liquefied petroleum gas (LPG) oxidation include: or natural gas or, exceptionally, light gas oil, can be used.
82 Environment Agency Guidance on the management of landfill gas Consideration should be given to the gas control solution to sustaining gas control, and options such as strategies, the cost of operation and sustainable intermittent extraction and flaring (which are often development issues when gas production is low. The preferred), or the use of low calorific flares and methane use of support fuels may not be the most appropriate oxidation provide other alternatives.
Table 7.4 Other options for landfill gas utilisation
Utilisation option Technology UK application Applications elsewhere Current applications Space heating Boilers Limited Limited Industrial process Kilns energy and heat Furnaces Moderate Limited Boilers Electricity generation Internal combustion engines Very common Very common Dual fuel compression engines Moderate Moderate Gas turbines Limited Common Steam turbines Limited Moderate Pipeline and gas Gas purification Limited Moderate grid use Future applications Electricity generation Fuel cells No commercial Pilot-scale application known Vehicle fuels Purified compressed fuel No commercial Pilot-scale application known
Table 7.5 Other landfill gas products and processes
Product/process Description/comments Project yield (m3/hour) Compressed methane Three stage compression to 35 barg. Trace components removed Sufficient for by absorption on carbon. Carbon dioxide removed by cellulose fuelling 13 acetate membrane separation. Methane product compressed to municipal 245 barg vehicles Liquefied methane Initial compression to 34 barg followed by removal of trace 1.6 components (as above). Carbon dioxide, water, oxygen and nitrogen removed by polyimide membrane separation. Product cooled and compressed to liquefy the methane.
Fuel cell Complex pretreatment of landfill gas to remove sulphur compounds 0.2 MWe and halides. The cleaned gas is fed to phosphoric acid fuel cell producing direct current electricity and process heat. Reticulation Process of gas clean-up stripping non-methane components No information to provide gas with higher calorific value. available Solid carbon dioxide Uses the triple point crystallisation process. 1.0 tonne/hour
MWe = MW electricity Barg = Bar gauge
Environment Agency Guidance on the management of landfill gas 83 7.4.8 Operation and maintenance The effectiveness of a landfill gas management system relies on consistent and thorough ongoing The operational procedures of a landfill gas maintenance. Failure to adopt a planned preventative management system should be set out in the Gas maintenance programme may lead to deterioration in Management Plan (see Chapter 3). The procedures the performance of the management system and should include: unacceptable emissions. A fully documented ● reference to the conceptual model providing monitoring, inspection and maintenance programme justification for system as built; should therefore be included in the Gas Management ● a system description including full as-built Plan. The programme should include: drawings together with a record of all subsequent ● detailed inspection programme with inventories changes to the as-built design, which should and frequencies including: include: – responsibilities for monitoring, inspection and – location specification and construction details maintenance; of all collection wells, collection pipework, – daily, weekly, monthly requirements, etc. for manifolds, valves, etc.; monitoring, inspection and maintenance; – details and process description of gas – procedures for documenting and reporting of extraction, utilisation, supplementary processing faults; and flaring plant; – procedures for implementing corrective – operational parameters for all elements of the actions; gas control system; ● a register of fault conditions and the corrective ● a complete set of all commissioning measurement actions taken to overcome the faults; data; ● details of routine repairs and replacements; ● operating instructions for each element of the gas ● review requirements for fault conditions and management system; repairs; ● commissioning into service and out of service ● an inventory of all replacement parts and contact procedures for each element of the gas details for relevant suppliers and manufactures. management system; ● a specification for routine operational monitoring Personnel responsible for the operation and for each element of the gas control system maintenance of the gas management system should including details of the parameters to be be fully conversant with operational procedures, and measured, the measurement precision required safety and maintenance programmes. and the frequency of measurement (see Chapters 5 and 8); ● a register of all routine adjustments, e.g. control valves; ● a record of all non-routine incidents; ● health and safety instructions for routine operation and further guidance on procedures to adopt in the event of accident or emergency (see Chapter 2). The successful operation of the system relies on routine monitoring and inspection. Therefore, all measurement data should be subject to careful inspection, checking and interpretation on an ongoing basis. In addition, the measurement data should be subjected periodically to a detailed appraisal and the results compared with the conceptual model and the objectives of the monitoring plan. Any inconsistencies should be investigated in consultation with the regulator and the conceptual model, and the Gas Management Plan should be modified to provide the necessary protection of the environment and human health.
84 Environment Agency Guidance on the management of landfill gas 8
Monitoring
This chapter sets out specific measures and Collection well monitoring is essential for the efficient techniques for the monitoring of landfill gas, based management of an extraction system. If monitoring on the regulatory requirements identified in Part A. A results show that the expected performance is not monitoring and sampling plan should be developed being achieved, the system should be balanced as a result of the conceptual site model as part of the and/or actions set out in the contingency plans Gas Management Plan as outlined in Section 3.3.3. implemented. Landfill gas monitoring should be undertaken for the 8.1.2 Monitoring wells following main components: Monitoring wells are those constructed within the ● source landfill for the purpose of monitoring landfill gas ● emissions concentrations and fluxes within the waste mass. This ● air quality is a requirement of the Landfill Regulations and ● meteorology. should enable the landfill gas to be characterised for each section of the landfill. 8.1 Source monitoring These wells are independent of the gas collection and extraction system, and are used as dedicated The aim of source monitoring is to characterise the monitoring points solely for the purpose of quantity and quality of the gas in each section of the ascertaining the composition of landfill gas and how landfill. Routine monitoring to determine the it responds to environmental conditions. composition of this gas is typically undertaken using portable hand-held instruments. These instruments Leachate monitoring or extraction wells may also be measure the bulk components within the landfill gas used for gas monitoring purposes and assisting with and associated physical parameters. Information on the establishment of gas production rates. If such the techniques that can be used for this routine monitoring points are used, however, they cannot be monitoring is given in guidance on the monitoring of regarded as comparable with, or even a substitute for, landfill gas published by the Institute of Wastes specifically designed gas monitoring points within the Management (IWM, 1998). waste mass. At depth, the measurements obtained may be affected by partial or complete blockages of Two different types of source monitoring points are perforations originally provided within the well. found on landfill sites: collection wells and monitoring Where these perforations are covered by leachate, the wells. These are discussed below. measurement may only be the concentration of gas As well as monitoring gas concentration, composition within the headspace. and pressure, gas flow rates should also be monitored Care should be exercised during the installation and in order to achieve sufficient control over gas maintenance of gas monitoring points to ensure that extraction and utilisation systems. Flow rates from these are appropriately sealed and do not provide active gas extraction wells can range from a few to routes through which air ingress can occur. several hundred cubic metres per hour. 8.1.3 Pressure monitoring 8.1.1 Collection wells Atmospheric pressure is an important parameter to Collection wells and associated manifolds are consider when checking source monitoring points. monitored to determine the effectiveness of the gas Atmospheric pressure should be measured regularly in extraction and collection system. Monitoring also order to aid understanding of gas pressure readings facilitates the balancing of the gas collection and within the waste body. Rapid drops in atmospheric extraction system. The recommended frequencies and pressure can result in the pressure of landfill gas determinands for monitoring collection wells are given in Table 5.4 (in Chapter 5).
Environment Agency Guidance on the management of landfill gas 85 being significantly above that of ambient atmospheric Much of this monitoring involves laboratory analysis pressure, resulting in possible migration. The of samples taken from the gas collection system. The monitoring of pressures within the waste mass gives monitoring is undertaken far less frequently and from an indication of the likelihood of gas migration fewer locations than the measurement of the bulk occurring. gases. The sampling point must be selected so that the gas is representative of the landfill or a particular The rate of change of pressure, both within the waste section of the landfill, e.g. taken from the gas main mass itself and atmospheric pressure, can significantly line. The sample should also be taken at a time when affect the migration of landfill gas. Monitoring of gas the gas collection system is at or near steady state pressure within the waste mass provides a way of conditions. measuring the following parameters. ● Total pressure – the pressure of landfill gas within the pores of the waste fill. Trends in total pressure 8.2 Emissions monitoring can be used to ascertain whether the gas Emissions monitoring on landfill sites will typically collection system is managing the gas generation consist of: within the fill. Total pressure within the waste mass can be subject to wide variations. ● surface emissions ● lateral emissions ● Rate of change of total pressure – this depends ● combustion emissions. on the rates of formation and dispersion of the gas, along with the changes in atmospheric 8.2.1 Surface emissions pressure. A rapid positive rate of change indicates Monitoring of methane gas emissions from the that the gas collection system does not have the surface of the landfill is undertaken to: capacity to manage the gas generated under falling atmospheric pressure conditions. A falling ● identify faults in the gas management system and rate of change of total pressure, coupled with air to prioritise the remediation required, ingress, indicates that the gas field is being over ● measure the total emissions from the site of pumped. methane, an important greenhouse gas involved in global warming. Pressure and composition monitoring within the waste requires the installation of permanent sampling A qualitative estimate of methane emissions through points distributed at selected locations. Infrequent a surface cap can be made using a hand-held measurement of gas concentrations and a failure to instrument such as a flame ionisation detector (FID). take barometric pressure readings at the time of However, very low flux cannot normally be detected sampling will make it difficult to demonstrate whether and localised on a landfill cap. Extensive research the gas readings were taken over falling, rising or suggests that the flux box is currently the most cost steady atmospheric pressure conditions. Atmospheric effective technique for the verification of the range of pressure should be monitored constantly or regularly surface emission sources typically found on a landfill (e.g. hourly) to make proper use of the pressure data. site. Flux boxes are enclosed chambers (see Figure This can be achieved using an automated weather 8.1) used to measure the rate of change in methane station. concentrations above a specific, small area of the landfill surface. By measuring the flux at a number of The required pressure monitoring frequencies are representative sampling points, an estimate can be given in Table 8.3. Both the parameters to be made of the total emissions from a zone. monitored and the frequency of monitoring should be set out in the Gas Management Plan. 8.1.4 Trace components A large number of substances are present at trace levels in landfill gas. These compounds contribute significantly to the potential odour and health impacts of the landfill gas. The regulator has identified a number of priority compounds that should be measured in landfill gas and has issued guidance on appropriate sampling and analytical methods for these compounds (Environment Agency, 2004f).
86 Environment Agency Guidance on the management of landfill gas 3 Flow rate, R (m /s) Atmospheric concentration C0 Balance valve Quick-fit DETECTOR connectors Activated carbon filter
DATA Perforated T-bar LOGGER
Flux box, volume V (m3) Flux box concentration
Landfill gas, flow rate L (m3/s) Figure 8.1 Schematic of a flux box for surface methane emissions measurement (adapted from Environment Agency, 2004d) Table 5.4 gives typical frequencies at which surface for monitoring landfill gas surface emissions emissions of landfill gas should be monitored. A (Environment Agency, 2004d). The key steps for method for the measurement of landfill methane surface emissions monitoring are shown in Figure 8.2. emissions using a flux box is set out in the guidance
Desk study Preliminary survey stage
Remediation Walkover survey
No Are methane surface Action plan emissions low?
• High concentration of • Low concentration of methane Yes methane above cap above cap
Design and plan • Within one year of capping flux box survey
Flux box survey • Regular re-survey
Remediation Formal report Gas management plan
No Yes Compliant with emissions Action plan standard?
Regular surface emission survey
Figure 8.2 Process of surface emissions monitoring
Source: Environment Agency, 2004d
Environment Agency Guidance on the management of landfill gas 87 The first step is a desk study to obtain the history of 8.2.2 Lateral emissions the waste disposal activities at the site and details of The monitoring of lateral emissions is undertaken the capping and gas control measures. Appendix 4 of using gas monitoring boreholes outside the perimeter the monitoring guidance (Environment Agency, of the deposited waste. These boreholes can be 2004d) contains a proforma which gives a located both on-site and off-site. They provide comprehensive guide of the parameters to assist in information on the movement of landfill gas below the characterisation of a site. the surface of the landfill from the waste mass. The The monitoring of emissions through a landfill cap monitoring of external boreholes is essential to has two stages: demonstrate the efficient management of gas within the site and to detect any gas migrating from the site. ● A preliminary stage using a portable FID to The key features of off-site monitoring boreholes are identify inadequacies in the gas containment and shown in Figure 8.3. collection system. ● A quantitative survey stage when the flux of Monitoring boreholes should remain sealed at all methane emitted through the intact cap is times to avoid the dilution of landfill gas with air. measured using an array of flux boxes. They should have a security cover to ensure that the valves cannot be tampered with. Using information from the desk study, a walkover of the site is conducted to identify any zones or features The background concentrations for methane and where a relatively high concentration of methane can carbon dioxide need to be established in consultation be detected. The zones should then be traversed in a with the regulator before landfilling begins. Details of systematic manner using a FID, held as close to the these levels and the appropriate action/trigger levels surface of the landfill as possible. A global positioning should be set out in the Gas Management Plan. If the system is frequently used to assist in the mapping of results of monitoring are at or above the appropriate the zones. High concentrations of methane at trigger levels, the regulator must be informed locations on the cap indicate inadequacies in the gas immediately and remedial action implemented within containment and collection system. Only when these an appropriately defined timescale. deficiencies have been remedied and there is a low The location and spacing of landfill gas monitoring concentration of methane above the surface is it boreholes is site-specific and depends on the likely appropriate to begin a quantitative survey of the risks posed by off-site gas migration. The risk varies surface flux. with: The information from the FID survey is then used in ● gas quality and volume conjunction with the results of the desk study to ● gas permeability of the wastes divide the site into monitoring zones where the ● site engineering works (e.g. control measures such surface characteristics are similar. A matrix of as site liners and caps) monitoring points is specified for each zone so that ● proximity of buildings and services the quantitative survey is representative of the total ● surrounding geology. surface of the site. The spacing of monitoring points must be considered Flux boxes are then placed at the selected locations as part of the risk assessment for the site and should and sealed against the ground surface. The rate at be based on the conceptual model. Off-site which methane enters the confined space is monitoring boreholes have historically been located measured using a FID and a flux for that location is relatively close to the edge of the waste fill. It is calculated. The individual measurements from all the recommended that boreholes are sited at least 20 locations are aggregated to estimate the emission rate metres from the boundary of the waste. Guidance on for the whole site or for a specific zone within the the spacing of monitoring boreholes is given in Table site. 8.1. However, the distances presented in Table 8.1 Field research has shown that low surface flux are only a guide, as the risk assessment is the primary emissions can be achieved by following current tool for determining the minimum required borehole practice for site capping and gas abstraction systems. spacing (this may be less than the minimum The emission standards reflect best practice indicated). conditions for permanently and temporarily capped zones (Environment Agency, 2004d). Where zones do not meet the emission standards, further remediation or improvement of the gas management system must be undertaken and the surface re-surveyed.
88 Environment Agency Guidance on the management of landfill gas Table 8.1 Guidance on typical off site monitoring borehole spacing
Site description Monitoring borehole spacing (m) Minimum Maximum Uniform low permeability strata (e.g. clay); no development within 250 metres 50 150 Uniform low permeability strata (e.g. clay); development within 250 metres 20 50 Uniform low permeability strata (e.g. clay); development within 150 metres 10 50 Uniform matrix dominated permeable strata (e.g. porous sandstone); no development within 250 metres 20 50 Uniform matrix dominated permeability strata (e.g. porous sandstone); development within 250 metres 10 50 Uniform matrix dominated permeability strata (e.g. porous sandstone); development within 150 metres 10 20 Fissure or fracture flow dominated permeable strata (e.g. blocky sandstone or igneous rock); no development within 250 metres 20 50 Fissure or fracture flow dominated permeable strata (e.g. blocky sandstone or igneous rock); development within 250 metres 10 50 Fissure or fracture flow dominated permeable strata (e.g. blocky sandstone or igneous rock); development within 150 metres 5 20
Note: The maximum spacings given in relation to the development relate to the zone of development and not the entire boundary. The monitoring frequencies required for off-site gas monitoring boreholes are shown in Table 5.4, but are subject to site-specific considerations (see Chapter 5). Before sampling using portable instruments or other techniques, atmospheric pressure and borehole pressure should be measured. If the pressure differential is large, it is an indication that gas is likely to be moving under advective pressure. If landfill gas is detected in a monitoring borehole, it is also likely that landfill gas will have migrated beyond the monitoring point. The lack of a positive pressure reading (relative to atmospheric pressure) when landfill gas is present in the borehole may indicate that landfill gas is migrating off-site through diffusive flow. The trigger levels for methane and carbon dioxide permitted in any off-site gas monitoring borehole, without remedial intervention, are given in Table 8.2. Trigger levels are compliance levels and, in order to meet them, action levels should be set at a level at which the operator can take action to remain compliant. Trigger levels form part of the PPC permit.
Figure 8.3 The features of a landfill gas monitoring borehole (adapted from IWM, 1998)
Environment Agency Guidance on the management of landfill gas 89 Table 8.2 Trigger levels for gas monitoring boreholes
Parameter Trigger concentrations (% v/v)
Methane 1 per cent above agreed background concentrations1 Carbon dioxide 1.5 per cent above agreed background concentrations2
1Based on 20 per cent of the LEL IPPC Horizontal Guidance Note H1 (Environment 2Based on 20 per cent of the 8-hour UK Occupational Exposure Agency, 2002c) provides a comprehensive list of EALs Standard (OES) for assessing the releases to air from a variety of processes. The EAL for an airborne compound relates to the concentration below which the substance in 8.2.3 Combustion monitoring question is considered to have no significant The Regulator has developed a series of guidance environmental impact. Most levels have been documents that specify the requirements for calculated as fractions of the values provided in HSE monitoring landfill gas flares and engines guidance on occupational exposure limits (OELs) (Environment Agency, 2004b; 2004e). The guidance (HSE, 2002). provides a standard set of monitoring methods that Air quality monitoring on landfill sites will typically allow for the collection of emissions data in a consist of: transparent and consistent manner. Further information on monitored emissions from enclosed ● odour monitoring landfill gas flares is presented in Appendix D of this ● particulate matter monitoring. document. 8.3.1 Odour monitoring The Agency guidance documents also provide a Odour is defined in the Agency’s technical guidance tiered approach to the formation of emission for the regulation of odour at waste management standards for landfill gas engines and flares. These facilities (Environment Agency, 2002a) as: standards comprise of a generic emission requirement based on best practice, combined with a stricter site- ● that characteristic property of a substance which specific risk based standard, where appropriate. makes it perceptible to the sense of smell; ● a smell whether pleasant or unpleasant; fragrant or stench. 8.3 Monitoring air quality The perceptibility of an odour depends on the The monitoring of air quality within and around concentration of that substance or mixture of landfill sites is becoming increasingly important. The substances in the atmosphere and, for each pure Agency has produced a number of Technical substance, there is a limiting concentration in air Guidance Notes, which provide important reference below which the odour is not perceptible. This is information relating to the monitoring of air quality known as the odour threshold of that substance. (applicable in England and Wales), including: Mixtures of more than one substance such as landfill ● M1 Sampling requirements for monitoring stack gas are more complex as the constituent gases can emissions to air from industrial installations interact; the odour threshold of the mixture is less (Environment Agency, 2002i); than (hypo-addition), greater than (hyper addition) or ● M2 Monitoring of stack emissions to air equal (complete addition) to the sum of the gases (Environment Agency, 2002j); individual intensities. ● M8 Environmental monitoring strategy – ambient air An odour can be described by four interlinked sensory (Environment Agency, 2002k); characteristics: ● M9 Monitoring methods for ambient air (Environment Agency, 2000d); ● odour concentration, i.e. the amount of odour ● M17 Monitoring of particulate matter in ambient air present in the air; around waste facilities (Environment Agency, ● hedonic tone, i.e. relative pleasantness or 2004g). unpleasantness; ● quality, qualitative attribute e.g. fruity; ● intensity, i.e. the perceived strength of the odour.
90 Environment Agency Guidance on the management of landfill gas The offensiveness of an odour is highly subjective. The Agency’s technical guidance for the regulation of The judgement of the offensiveness of an odour is odour at waste management facilities in England and dependent upon factors such as race, gender, age, Wales (Environment Agency, 2002a) details how occupation, health and previous history of odour odour monitoring should be undertaken at a landfill experiences. Odours are often expressed in odour site. Most of this guidance is relevant to sites units (ou/m3), with one odour unit being the regulated under the PPC regime. concentration at which 50 per cent of an odour panel On-site odour assessments should be carried out, as detect the odour. far as possible, in a distal to proximal direction, i.e. The monitoring of odour is undertaken to fulfil a from the furthest point away from the site relative to number of differing objectives including: the wind direction towards the site boundary or onto the site itself, downwind of the site and in a proximal ● the development of input data for risk assessment to distal direction up wind of the site. The persistence and predictive dispersion modelling of the odour, together with its location from the site ● development of gas monitoring plan boundary, should be noted. ● prioritisation of odour sources for mitigation or abatement ● selection of odour abatement measures ● assessment of the effectiveness of odour abatement and mitigation measures.
Application of practical measures during normal operations
Perimeter and meteorological monitoring
Odour observed Odour not observed
Identify source of odour
Odour from a discrete mass of waste/area Odour from operational activity No action required
Apply additional cover, re-balance gas field Re-balance gas field
Monitor (perimeter) effectiveness of Mitigation
Odour observed Odour not observed
Introduce additional cover, No further action re-balance gas field required
Monitor (perimeter) effectiveness of Mitigation
Odour observed Odour not observed
Suspend odour causing activity until conditions improve
Persistent odours and reoccuring odours observed
Evaluate and alter operational controls. Assess potential application of alternative mitigation measures
Figure 8.4 Typical odour action plan
Environment Agency Guidance on the management of landfill gas 91 Subjective monitoring can be supported by Further information on monitoring of particulate techniques such as olfactometry, gas chromatography matter in ambient air around waste facilities is given and mass spectrometry. Collected samples of air in Technical Guidance Document M17 (Environment should be assessed for the strength of an odour and Agency, 2004g). the potential source. Olfactometry involves the presentation of the samples of air at various levels of dilution to an odour panel. These methods are 8.4 Meteorological monitoring covered in more detail in Horizontal Odour Guidance Annex III of the Landfill Directive requires that H4 (Environment Agency, 2003b). meteorological data is collected at the frequencies As an alternative to olfactometry, measurement of a given in Table 8.3. This requirement will typically be selected ‘marker’ compound in the atmosphere (e.g. included in a PPC permit and the suggested hydrogen sulphide) may be used as a surrogate meteorological monitoring in the Landfill Directive is measure for odour. Portable monitoring instruments required by the Agency. The UK is required to report are available that can measure hydrogen sulphide on a three-yearly basis to the European Commission down to parts per billion concentrations and a large on the methods used to collect these data. area can be covered in a relatively short time. This Meteorological data should ideally be collected using technique has been used successfully to pinpoint an on-site weather station with an automated logging odour and landfill gas escapes on a number of UK capability. In some circumstances, it may be landfills. It is also possible to select specific organic appropriate to obtain some or all of these data from a components to be monitored, e.g. VOCs, local meteorological station. organosulphur compounds and aromatic hydrocarbons. Measuring the concentration of the carrier gas (methane) can also be an effective way of 8.5 Monitoring procedures identifying the extent of the odour plume. Sampling and analytical methods used in monitoring Odour action plans should be developed to engage a must be selected to ensure reliable and accurate series of specific actions or odour mitigation measures results. A description of the measurement techniques in response to a particular event or anticipated result. and sampling strategy, in addition to the analytical An example schematic diagram of an odour action and testing schedules, should be included in the plan is shown in Figure 8.4. monitoring section of the Gas Management Plan. 8.3.2 Particulate monitoring Table 8.4 summarises the relationship between the monitoring purpose and the type of instrument to be Particulates can be present in the landfill gas and are used. also generated by landfill gas combustion plant.
Table 8.3 Meteorological monitoring suggested by the Landfill Directive
Landfill Directive Operational and restored Aftercare phase Requirements phases
Volume of precipitation Recorded daily, reported monthly. Monthly average based on the sum of daily records. Maximum and minimum Recorded daily at 15:00 GMT, Monthly average maximum and temperature, recorded at reported monthly minimum temperatures based on 15:00 GMT daily measurments taken at 15:00 GMT Wind speed and direction Recorded continuously, reported Not required. yearly as a wind rose. Evaporation Daily Monthly average based on the sum of daily records. Barometric pressure Daily Monthly average based on the sum of daily records. Atmospheric humidity, Daily Monthly average based on the recorded at 15:00 GMT sum of daily records. GMT = Greenwich Mean Time
92 Environment Agency Guidance on the management of landfill gas Table 8.4 The relationship between monitoring purpose and instrument types (adapted from IWM, 1998)
Purpose Monitoring location Measured parameters Instrument type
Monitoring for gas Ground surface services Flammable gas (methane), Portable during a surface survey services manholes, carbon dioxide and oxygen search bar holes concentration Pressure, temperature and flow. Monitoring for gas Gas monitoring Flammable gas (methane), Portable outside the waste borehole or probe carbon dioxide and oxygen OR concentration Pressure, Fixed for continuous temperature and flow. monitoring with telemetry (optional) Monitoring the gas in Gas or leachate Flammable gas (methane), Portable the waste or within a extraction well, carbon dioxide and oxygen OR gas collection system Knock-Out-Pot (gas concentration Pressure, Fixed for continuous de-watering plant), temperature, flow monitoring with gas collection pipes calorific value and moisture telemetry (optional) Monitoring in a gas Gas flare Temperature (continuous) Fixed for continuous thermal destruction unit Flammable gas (methane), monitoring with arbon dioxide and oxygen telemetry (optional) oncentration Pressure, temperature and flow (periodic) Monitoring in a gas Engine Flammable gas (methane), Portable utilisation plant carbon dioxide and oxygen OR concentration Pressure, Fixed for continuous temperature, flow, calorific monitoring with value and moisture telemetry (optional) Detailed gas analysis1 Sample of gas Gas composition and Fixed or transportable concentration of its laboratory instruments components (including priority (e.g. GC–MS) trace components) Moisture
1 The Landfill Directive requires that any analysis must be undertaken by a competent laboratory.
Landfill gas sampling errors often occur due to instrument should be set at zero when in use and in dilution of the gas source – either because gas the absence of landfill gas. volumes are too small or due to leaks in the sampling The range of instruments that can be used to monitor system. landfill gas is extensive and increasing as new Sampling pipework must be flushed with the gas detection systems are developed and old systems mixture to be analysed before sampling to ensure air refined. Such instruments generally fall into three dilution during sampling does not occur. However, if categories: the sampling of small volumes is unavoidable, this ● portable hand-held instruments with a self- fact must be recorded. contained power supply Surface openings of any monitoring points should be ● semi-portable instruments which are battery or kept sealed before and during sampling to avoid mains driven dilution with air. When the volume of gas to be ● in situ monitoring units (usually mains powered). sampled is small compared with the amount of gas When taking field measurements with portable being drawn through the instrument, a peak reading instruments, it is important to: will be obtained which will fall to a steady reading, proportional to the effectiveness of the seal of the ● record the location and conditions in which the sampling system and the volume of gas entering the measurements are being carried out (IWM, 1998); sampling volume. Both the peak and steady state ● train staff to take field measurements concentrations should therefore be noted. An ● adhere to site health and safety requirements.
Environment Agency Guidance on the management of landfill gas 93 Portable instruments tend to be relatively light, robust Interpretation of data should include exception reports under most field conditions, possess integral pumps or to highlight where deviations are occurring. Should any are aspirated by hand, and provide a direct readout. values at or above the agreed action/trigger levels be However, portable instruments are often confined to a recorded, then the regulator must be informed and a limited range of parameters with varying degrees of previously agreed course of remedial action – as set out sensitivity. Several portable instruments, including in the Gas Management Plan – implemented. thermal conductivity and catalytic detectors, are Technical Guidance Note M8 (Environment Agency, equipped with more than one detection system. When 2002k) details the statistical analysis and reporting of taking field measurements with portable instruments, results required for air quality data in England and the equipment should have in-line filters installed to Wales. It emphasises the importance of summarising enable any condensate, moisture and dust to be data in such a way as to allow meaningful trapped. These should be checked regularly. interpretation. The raw monitoring data can be Semi-portable units may measure a wider range of manipulated to produce simple tables and graphs to parameters at a greater sensitivity than portable display the data. instruments. However, these units tend to be When reporting large amounts of raw data, the use of cumbersome and fragile, and often need an external statistics is recommended so that the data are described power source. In situ instruments have dedicated by a limited set of numerical values. Common statistical functions and combine some of the attributes and analyses include: drawbacks of both portable and semi-portable instruments. ● ranking the raw data –- placing the data set in order from the lowest to the highest value Sensors used to measure the concentrations of landfill obtained; gas include infra-red, catalytic oxidation, thermal ● obtaining the range of the raw data – the conductivity, flame ionisation, semi-conductor, difference between the highest and lowest ranked paramagnetic and electrochemical gas detectors. More data values; detailed information about these analysers is given in ● obtaining the median value – the middle value of Appendix F. Any equipment used in the monitoring of the ranked data; landfill gas must be calibrated and serviced in ● obtaining the quartile values –- by dividing the accordance with the manufacturer’s recommendations ranked data into four; to ensure that accurate measurements are taken. ● obtaining the decile value – by dividing the Records must be kept of the services and calibration for ranked data into ten; each instrument. ● use of frequency distributions – group ranking of Check samples of gas should be analysed by a the data to enable their distribution to be plotted laboratory on an occasional basis to confirm the results as a frequency curve or a histogram; of on-site analysis. ● measures of central tendency – common measures include the arithmetic mean (average), the median (middle number of a ranked data set), 8.6 Data analysis and reporting the mode (the most frequently occurring number) and the geometric mean (logarithmic mean). Operators are typically required to provide the regulator with annual reports of the data obtained monitoring Data can also be reported in a time-series plot, which is landfill gas. These reports should: a useful method for showing fluctuations of different pollutants at the same site due to diurnal (daily) or ● contain raw and aggregated data, charts and seasonal effects and for showing possible anomalous trends; results within the data set. ● demonstrate compliance with the conditions of the landfill permit; Detailed information relating to the use of statistics in ● provide an interpretation of the data, including analysing air quality data is given in Section 12 of comparison with the objectives of the Gas Technical Guidance Note M8 (Environment Agency, Management Plan and a periodic review of the 2002k). conceptual site model; ● include proposed revisions of the Plan and the model in light of this review. The data will form part of the public register and should be supplied in electronic format.
94 Environment Agency Guidance on the management of landfill gas Guidance on the management of landfill gas
Appendices
Environment Agency Guidance on the management of landfill gas 95 A
Appendix A: Identified trace components in landfill gas Chemical Chemical Chemical
(1-methylethyl)benzene 1,2,4-trimethylcyclohexane 1-butene (1-methylethyl)cyclohexane 1,2,4-trimethylcyclopentane 1-chloro-1,1-difluoroethane 1-(ethenyloxy)-butane 1,2-dichloro-1,1,2,2-tetrafluoroethane 1-chloro-1-fluoroethane 1-(ethylthio)-butane 1,2-dichloro-1-fluoroethane 1-chloropropane 1,1,1,2-tetrachloroethane 1,2-dichlorobenzene 1-decene 1,1,1,2-tetrafluorochloroethane 1,2-dichloroethane 1-ethenyl-3-ethylbenzene 1,1,1-trichloroethane 1,2-dichloroethene 1-ethyl-2,3-dimethylbenzene 1,1,1-trichlorotrifluoroethane 1,2-dichlorotetrafluoroethane 1-ethyl-2-methylbenzene 1,1,1-trifluoro-2-chloroethane 1,2-dimethyl-3-(1-methylethyl) 1-ethyl-2-methylcyclohexane 1,1,1-trifluorochloroethane 1,2-dimethylcyclohexane 1-ethyl-2-methylcyclopentane 1,1,2,2-tetrachloroethane 1,2-dimethylcyclopentane 1-ethyl-3-ethylbenzene 1,1,2,2-tetrafluoroethane 1,2-dimethylcyclopropane 1-ethyl-3-methylcyclohexane 1,1,2-trichloro-1,2,2-trifluoroethane 1,3,5-trimethyl cyclohexane 1-ethyl-3-methylcyclopentane 1,1,2-trichloroethane 1,3,5-trimethylbenzene 1-ethyl-4-methylcyclohexane 1,1,2-trifluoro-1,2,2-trichloroethane 1,3,5-trimethylcyclohexane 1-heptene 1,1,2-trifluoro-1,2-dichloroethane 1,3-butadiene 1-hexene 1,1,2-trifluoro-1-chloroethane 1,3-dichlorobenzene 1-methyl-2-propylbenzene 1,1,3-trimethylcyclohexane 1,3-dimethylcyclohexane 1-methyl-2-propylcyclopentane 1,10-undecadiene 1,3-dimethylcyclohexane (cis) 1-methyl-3-propylbenzene 1,11-dodecadiene 1,3-dimethylcyclohexane (trans) 1-methyl-4-(1-methylethyl) benzene 1,1-chlorofluoroethane 1,3-dimethylcyclopentane 1-methyl-4-(1-methylethyl) 1,1-dichloroethane 1,3-dimethylcyclopentane (trans) 1-methyl-4-propylbenzene 1,1-dichloroethene 1,3-dioxolane 1-methylpropylbenzene 1,1-dichlorotetrafluoroethane 1,3-pentadiene 1-octene 1,1-difluoro-1-chloroethane 1,4-dichlorobenzene 1-pentanethiol 1,1-dimethylcyclopropane 1,4-dimethylcyclohexane 1-pentene 1,1-thiobispropane 1,4-pentadiene 1-phenyl-1-propanone 1,1-trichloroethane 1,6-dimethylnaphthalene 1-propanethiol 1,2,3,4,6,7,8-HpCDD 1,6-heptadiene 1-propanol 1,2,3,4,6,7,8-HpCDF 1,8-nonadiene 1-undecene 1,2,3,4,7,8,9-HpCDF 1,9-decadiene 2 ethynyl phenol 1,2,3,4,7,8-HxCDD 1234678 H7CDF 2(2-hydropropoxy)propan-1-ol 1,2,3,4,7,8-HxCDF 1234679 H7CDD 2(methylthio)propane 1,2,3,4-tetrachlorobenzene 123478 H6CDD 2,2-difluoropropane 1,2,3,6,7,8-HxCDD 123478 H6CDF 2,2-dimethylbutane 1,2,3,6,7,8-HxCDF 1234789H7CDF 2,2-dimethylpentane 1,2,3,7,8,9-HxCDD 123678 H6CDD 2,2-dimethylpropanoic acid 1,2,3,7,8,9-HxCDF 123678 H6CDF 2,3,3-trimethylpentane 1,2,3,7,8-PeCDD 12378 P5CDF 2,3,4,6,7,8-HxCDF 1,2,3,7,8-PeCDF 123789 H6CDD 2,3,4,7,8-PeCDF 1,2,3-trichlorobenzene 123789 H6CDF 2,3,4-trimethylhexane 1,2,3-trimethylbenzene 12379 P5CDD 2,3,4-trimethylpentane 1,2,4-trichlorobenzene 1-butanethiol 2,3,7,8-TCDD 1,2,4-trimethylbenzene 1-butanol 2,3,7,8-TCDF
96 Environment Agency Guidance on the management of landfill gas Chemical Chemical Chemical
2,3-dimethylheptane 3-(ethylthio)propanal butyl acetate 2,3-dimethylpentane 3,3-dimethylpentane butyl benzene 2,4,4-trimethylpentane 3,5-dimethyloctane butyl butyrate 2,4,6-trimethylheptane 3-carene butyl cyclohexane 2,4-dimethylheptane 3-ethyl-4-methylheptane butyl ester 2,4-dimethylhexane 3-ethylhexane butyl ethanoate 2,5-dimethylheptane 3-ethylpentane butyl ethyl trisulphide 2,5-dimethylhexane 3-methyl butan-2-ol butyl formate 2,5-dimethylpentene 3-methyl pentan-2-ol butyl methyl trisulphide 2,6-dimethylheptane 3-methyl-1-butanol butyl propyl trisulphide 2,6-dimethylnonane 3-methyl-2-butanone butyl trisulphides 2,6-dimethyloctane 3-methyldecane butylbenzene 2,6-dimethynonane 3-methylheptane butylpropyldisulphide 234678 H6CDF 3-methylhexane butynes 23478 P5CDF 3-methylnonane butyric acid 2379 T4CDD 3-methyloctane camphene 2379 T4CDF 3-methylpentane camphor 2-butanethiol 3-methypentane carbon disulphide 2-butanol 3-pentanol carbon monoxide 2-butanone 4-carene carbonyl sulphide 2-butene 4-methyl-1-hexene carene 2-butoxy ethanol 4-methyl-2-pentene (e) chlorobenzene 2-chloro-1,1,1-trifluoroethane 4-methyldecane chlorodifluoromethane 2-ethyl-1,3-dimethylbenzene 4-methylheptane chloroethane 2-ethyl-1-butanol 4-methylnonane chloroethene (vinyl chloride) 2-ethyl-1-hexanol 4-methyloctane chlorofluoromethane 2-ethyl-cycloheptanone 5-methyldecane chloromethane 2-furanmethanol acenaphthene chloromethylbenzene 2-hexanone acetaphenone chloropropene 2-methyl-1,3-butadiene acetone chlorotrifluoroethene 2-methyl-1-butane acetonitrile chlorotrifluoromethane 2-methyl-1-butene a-chlorotoluene chlorotrifluoromethene 2-methyl-1-pentene amyl acetate (mixed isomers) chrysene 2-methyl-1-propanethiol amyl alcohol cis-1,2-dichloroethene 2-methyl-1-propanol amyl mercaptan cyclobutane 2-methyl-1-propene anthracene cycloheptane 2-methyl-2-propenoic acid arsenic cyclohexane 2-methylbutane benzealdehyde cyclohexanone 2-methyldecane benzene cyclopentane 2-methylheptane benzo(a)anthracene cyclopentanone 2-methylhexane benzo(a)pyrene cyclopentene 2-methylnonane benzo(b)fluoranthene decahydro-4,8,8-trimethyl-9- 2-methyloctane benzo(ghi)perylene methylene- (1a,3aß,4a,8aß)]-1,4- 2-methylpentane benzo(k)fluoranthene methanoazulene 2-methylpropane benzoic acid decahydronaphthalene 2-methylpropylbenzene benzothiazole decamethylcyclopentasiloxane 2-methylpropylcyclohexane biphenylene decanal 2-pentanone bromochlorodifluoromethane decanhydronaphthalene 2-pentene bromochlorofluoromethane dibromochloromethane 2-propanethiol bromodichloromethane dibutyl sulphide 2-propanol bromoethane dibutyl trisulphide 2-propenal butanal dichlorobenzene (mixed isomers) 2-propene-1-thiol butane mercaptan dichlorobutene 2-propyl thiophene butanoic ethyl ester dichlorodifluoromethane 3 ethynyl phenol butene dichlorofluoromethane
Environment Agency Guidance on the management of landfill gas 97 Chemical Chemical Chemical 188 2-propenal 243 bromoethane 296 dibutyl trisulphide dichloromethane ethylmethyl trisulphide methyl ether diethyl disulphide ethylmethylcyclohexane methyl ethyl butanoate diethyl phthalate ethylpentane methyl ethyl disulphide diethyl sulphide ethylpropyl disulphide methyl ethyl ketone diethylbenzene ethylpropyl trisulphide methyl ethyl propanoate di-isooctyl phthalate ethylvinyl benzene methyl furan dimethoxy methyl propanoate ethyne methyl isobutyl carbinol dimethyl cyclohexane fluoranthene methyl isobutyl ketone dimethyl cyclopentane fluorene methyl isobutyrate dimethyl disulphide formic acid methyl isopropyl disulphide dimethyl ether furan methyl isopropyl ketone dimethyl ethyl methanoate furfural methyl isovalerate dimethyl furan HA1334 methyl naphthalene dimethyl pentan-3-one HA1335 methyl pentanoate dimethyl styrene HA1343 methyl propanoate dimethyl sulphide HA1344 methyl propyl disulphide dimethyl tetrasulphide HA1352 methyl propyl ethanoate dimethyl trisulphide HA1353 methyl vinyl ketone dimethylbutane heneicosane methyl-4-isopropenylbenzene dipropyl ether heptachlorodibenzodioxin methylal dipropyl sulphide heptadecane methylcyclobutane dipropyl trisulphide heptadibrodibenzofuran methylcyclohexane dodecamethylcyclohexasiloxane heptyl mercaptan methylcyclopentane dodecene hexachlorobenzene methylcyclopropane eicosane hexachlorodibenzodioxin methylenecyclohexane ethanal (acetaldehyde) hexadecane methylethyl cyclohexane ethane hexadibrodibenzofuran methylethyl sulphide ethanethiol hexadiene methylpropyltrisulphide ethanoic acid (acetic acid) hexamethylcyclotrisiloxane methylthioethane ethanol hexamethyldisiloxane naphthalene ethene hexanal n-butane ether hexyl methanoates n-butanol ethyl 2-methyl butyroate hydrogen n-butyl disulphide ethyl acetate hydrogen chloride n-butyl propionate ethyl alcohol hydrogen cyanide n-decane ethyl butyrate hydrogen fluoride n-decene ethyl caproate hydrogen sulphide n-dodecane ethyl cyclohexane indeno(123cd)pyrene n-heptane ethyl cyclopentane isobutane n-hexane ethyl dimethyl propanoate isobutyl formate n-hexanol ethyl ethanoate limonene n-hexyl mercaptan ethyl isopropyl disulphide l-propanol n-nonane ethyl isovalerate m-cresol n-octane ethyl methyl ether mercury nonadecane ethyl n-propyl disulfide methanal (acetaldehyde) nonanal ethyl pentanoate methanethiol nonene ethyl propionate methanol n-pentane ethyl toluene methyl 2-methyl butanoate n-propane ethylbenzene methyl 2-methyl propenoate n-propyl acetate ethylcyclohexane methyl acetate n-propyl butyrate ethylcyclopentane methyl butyl disulphide n-tetradecane ethylcyclopropane methyl butyrate n-tridecane ethylene oxide methyl caproate n-undecane ethylisobutyldisulphide methyl cyanide n-undecene ethylmethyl disulphide methyl cycloheptane OCDD (octachlorodibenzodioxin)
98 Environment Agency Guidance on the management of landfill gas Chemical Chemical
OCDF propyl propionate o-cresol propylbenzene octabromodibenzofuran propylthiophene octadecane propyltoluene octadiene pyrene octamethylcyclotetrasiloxane sec-butyl alcohol octanal sec-butylbenzene PCB 101 styrene PCB 118 sulphur dioxide PCB 126 sulphuric acid PCB 138 t-butyl alcohol PCB 153 t-butylbenzene PCB 169 terpenes PCB 180 tetrachlorodibenzodioxin PCB 28 tetrachloroethane PCB 52 tetrachloroethene PCB 77 tetrachloromethane p-cresol tetradecane p-cymenyl tetradibrodibenzofuran pentachlorobenzene tetrafluorochloroethane pentachlorodibenzodioxin tetrahydro-2-furanmethanol pentadecane tetrahydrofuran pentadibrodibenzofuran tetramethylbenzene pentanal tetramethylcyclohexane pentene thiophene pentyl benzene thujene pentyl methanoate toluene pentyl trisulphide trans-1,2-dichloroethene phellandrene tribromomethane phenanthrene trichloroethene phenol trichlorofluoromethane pinene trichloromethane propadiene trifluorobenzene propan-2-one trimethyl cyclopentane propanal trimethylhexane propanoic acid trimethylsilanol propene vinyl toluene propionic acid xylene propyl butyl disulphide α-pinene propyl cyclohexane ß-cymene propyl methyl propanoate ß-pinene propyl methyl trisulphide γ-terpinene Source: revised from Environment Agency, 2002e
Environment Agency Guidance on the management of landfill gas 99 B
Appendix B: Compositional comparison of gas sources
A compositional comparison of typical landfill gas with other gas sources
Compound Landfill gas Marsh gas Natural gas Mine gas
Methane 20–65 11–88 17–97 22–95 Ethane Trace 0.7–16 3–8 Propane Trace 0.4–7.9 1–4 Butane Trace 0.1–3.4 0–1 Carbon dioxide 16–57 0–9.5 0.2–6.0 Carbon monoxide 4.7 0–10 Nitrogen 0.5–37 3-69 0.1–22 1–61 Helium/argon Often removed Present Source: Latham, 1998. Note: Figures are based on %v/v.
Mode of origin
Biogenic gases Thermogenic gas landfill gas mine gas marsh gas natural gas sewer gas
Source of gas
Natural Man-made coal measures landfills peat and organic soil deposits slurry spreading marsh sediments leaking lagoons marine and freshwater sediments silage liquor petroleum deposits cesspit soakaways sewers spillages chemical reactive soils coking ovens
100 Environment Agency Guidance on the management of landfill gas C
Appendix C:
Effects of CO2 on the flammable limits of methane A 0 100
Zone of flammable 10 gas mixtures 90
20
80 79% air 16% O2
30 70 Min air req. =68% =14.3% O2
e percentage by volume 40 um 60 vol Air
ge by a Methanet 50 50 Percen
60 40
100% margin 7.2% O2
70 30
200% margin 4.6% O2 Current practice 80 20 shutdown level 4% O2
Current practice alarm level 2% O 2 10 10
NOTES NOTES 20 C 30 0 1. Undiluted LFG mixtures 20 30 C 40 50 60 70 1. Undiluted LFG mixtures 80 D (i.e.no air) fall on line CD 76% CO (i.e.no air) fall on line CD Carbon Dioxide 2. Dilution of LFG by air follows a Percentage by volume 2.straight Dilution line ofsuch LFG as byCA air follows a 2 24% 3. The straight LFG air linemixture such becomes as CA 3.flammable The LFG when air mixture the composition becomes CH 4 falls flammable within the whenshaded the zone composition Source: falls Cooper within et. al.,the1993 shaded zone
Environment Agency Guidance on the management of landfill gas 101 D
Appendix D: Average flare emissions data
Averaged emissions data from a range of landfill gas flares (all mg/Nn3)
Measured value Determinand Site AB CDE FGHI J
Inlet gas Methane (%)b 55 56 45 44 33 54 36 39 36 46 Carbon dioxide (%)b 39 41 31 32 30 43 30 34 23 37 Oxygen (%)b 0.4 <0.1 0.2 4.4 7.0 0.9 6.2 0.6 7.0 1.8 Nitrogen (%)b 5.0 3.2 24 20 30 1.9 28 21 34 15 Hydrogen sulphide (ppm)b <5 587 23 30 85 1416 33 89 5 18 Carbon monoxide (ppm)b <2 11 24 45 786a 40 530a 56 36 194 Emissions Temperature (oC)b 513 956 588 986 1208 1162 992 849 738 862 Oxygen (%)b 17.3 12.6 15.3 11.0 5.1 6.4 11.5 12.4 14.3 11.5 Carbon dioxide (%)b 2.6 5.7 4.1 8.6 14.7 12.4 8.3 5.8 5.3 8.3 Moisture (%)b 3.1 5.6 3.6 15.0 13.0 16.3 8.4 15.3 7.2 12.2 Carbon monoxide (mg/m3)c 1,042 617 2178 27 32 34 253 34 99 <2 Oxides of nitrogen 3 c (as NO2) (mg/m ) 75 111 43 92 99 149 82 59 63 14 Total VOCs (as C) (mg/m3)c 21 3 2 <2 2 <2 10 6 17 <2 Hydrogen chloride (mg/m3)c 36 9.5 4.6 7.4 11 4.2 36 7.4 4.9 16.2 Hydrogen fluoride (mg/m3)c 21 2.5 0.4 2.5 0.7 1.6 7.8 2.5 0.5 0.5 Sulphur dioxide (mg/m3)c 482 239 63 30 43 359 181 61 58 83
a Denotes the result may be affected by possible interference due to the presence of hydrogen.
b On-site measurement
c Averaged emission by laboratory analysis (at reference conditions of 3% O2, 273K, 101.3kPa, dry).
Source: Environment Agency, 2004b
102 Environment Agency Guidance on the management of landfill gas E
Appendix E: Example emergency procedures
Example emergency procedures: methane (flammable gas) Level of Location Action to be taken by Action to be taken by methane the person undertaking landfill manager the monitoring
At 100 ppm Monitoring No action No action (0.2% LEL) boreholes
At 100 ppm In buildings Immediately inform landfill Review monitoring frequency and (0.2% LEL) or servicess manager. Take gas sample(s) as prepare for additional monitoring measures. within 250 soon as practicable for confirmatory gas meters chromatography analysis.
At 1,000 ppm Monitoring Immediately inform landfill Inform the regulator. (2% LEL) boreholes manager Review control measures.
At 1,000 ppm In buildings Immediately inform landfill With regard to detection in buildings, inform (2% LEL) or services manager. Take gas sample(s) as the regulator and other appropriate within 250 soon as practicable for confirmatory authorities. Immediately review building meters gas chromatography analysis. monitoring frequency.
At 5,000 ppm Monitoring Immediately inform landfill Inform the regulator. Assess the landfill (10% LEL) boreholes manager. Take gas sample(s) as gas risk. Consider a revised programme and rising soon as practicable for analysis by for landfill gas control. confirmatory gas chromatography. Increase frequency of monitoring probes.
At 5,000 ppm In buildings Immediately inform landfill Inform regulator and other relevant (10% LEL) and services manager. Take gas sample(s) as authorities. Implement continuous and rising within 250 soon as practicable for monitoring. Prepare for implementation meters confirmatory analysis by gas of evacuation procedure. Consider the chromatography. installation of audible alarms.
At 9,000– In buildings Immediately inform landfill Instruct monitoring personnel on 10,000 ppm and services manager. Receive instructions evacuation/ventilation/isolation of ignition (18–20% LEL) within 250 on carrying out evacuation sources, etc. Immediately inform regulator meters procedure, ventilating building(s) and other relevant authorities. Send out and switching off sources of additional assistance to site. Prepare for ignition. rapid implementationof evacuation procedures.
At 10,000 ppm Monitoring Immediately inform landfill Immediately inform regulator and other (20% LEL) boreholes manager. Take gas sample(s) as relevant authorities. Assess the risk soon as practical for confirmatory Immediately consider the monitoring of analysis by gas chromatography. adjacent buildings.
Above 10,000 In buildings Immediately inform landfill Immediately inform regulator and other ppm (20% LEL) and services manager and follow evacuation relevant authorities. Immediately send out within 250 procedure. additional assistance to site. Immediately meters follow evacuation procedures.
Environment Agency Guidance on the management of landfill gas 103 Example emergency plan: carbon dioxide Level of Location Action to be taken by Action to be taken by carbon the person undertaking landfill manager dioxide the monitoring
0.4% v/v In buildings Immediately inform landfill Inform regulator and other relevant and services manager. Carry out an immediate authorities. Make preparations for within 250 check on the environmenta evacuation. metres conditions within the building, e.g. has a poorly ventilated room been occupied by people or animals, or has a gas appliance been used without a fume extraction (if it has, the occupier must be immediately told not to use the gas appliance and the gas supplier informed as soon as practicable). Immediately ventilate the building. Take gas sample(s) as soon as practicable for confirmatory gas chromatography analysis.
0.5% v/v In buildings Check on environmental Inform regulator and other relevant and services conditions as for 0.4% CO2 level. authorities. If CO2 levels reach 0.5% v/v within 250 Vent building and check gas within 24 hours, then follow metres concentrations again. Leave gas evacuation procedure. monitors in building for 24 hours to give audible warning of gas build up. Take gas sample(s) as soon as practicable for confirmatory gas chromatography analysis.
1.5% v/v Monitoring Immediately inform landfill Inform the regulator and other relevant boreholes manager. Take gas sample(s) as authorities. Assess monitoring date soon as practical for confirmatory against background levels. Undertake a gas chromatography. gas survey of neighbouring buildings. Increase the monitoring frequency for the monitoring boreholes to daily, as necessary.
1.5% v/v In buildings Immediately inform landfill Immediately inform the regulator and and services manager and follow evacuation other relevant authorities. Immediately within 250 procedure. send out additional assistance to site. metres Immediately follow evacuation procedures.
104 Environment Agency Guidance on the management of landfill gas F
Appendix F: Characteristics of various gas sensors
Type Gas Advantages Disadvantages
Infra-red Methane, Fast response Prone to zero drift. carbon dioxide Can be used to measure specific Pressure sensitive and other gases in gas mixtures. Temperature sensitive hydrocarbons Simple to use Moisture sensitive Wide detection range Majority of instruments sensitive to (ppmv to 100% v/v) hydrocarbon bond only, not specifically Less prone to cross interference to methane – in presence of specific with other gases than other organic compounds can cause sensors. interference. Cannot be ‘poisoned’. Optics sensitive to contamination Can be incorporated into (condensate, particulates) intrinsically safe instruments. Gas sample passes unchanged through the sensor.
Flame ionisation Methane Highly sensitive Will not work in oxygen-deficient Flammable (usual range 0.1–10,000 ppmv) environment. gases and Fast response Accuracy is affected by presence of other vapours gases such as carbon dioxide, hydrogen, minor constituents of landfill gas, and water vapour. ‘Blind test’ – responds to any flammable gas. Limited detection range Gas sample destroyed
Electrochemical Oxygen, Low cost Limited shelf life hydrogen Usual detection range Requires frequent calibration. sulphide and 0–25% v/v against various gases Can lose sensitivity due to moisture, carbon corrosion and poisoning. monoxide Poor performance against crosscontamination with other components of landfill gas
Paramagnetic Oxygen Accurate Prone to drift and gas contaminants. Robust Expensive No interference from other gases Responds to partial pressure and not to concentration.
Catalytic Methane Fast response Accuracy affected by presence of other oxidation Flammable Low detection range flammable gases. (pellistor) gases and (0.1–100% LEL) Readings inaccurate in oxygen-deficient vapours Responds to any flammable gas. environment (<12% v/v) Prone to ageing, poisoning and moisture. Not possible to notice sensor deterioration Gas sample destroyed during measurement.
Environment Agency Guidance on the management of landfill gas 105 Type Gas Advantages Disadvantages
Thermal Methane Fast response to any flammable Accuracy affected by the presence of conductivity Flammable gas other flammable gases, carbon dioxide gases and Full detection range and other gases with the same thermal vapours (0–100% v/v) conductivity. Independent of oxygen level Sensitivity too poor for use in safety Can be combined with other checks detectors. Errors at low concentrations
Semiconductor Mainly toxic Good selectivity for some toxic Lack of sensitivity to combustible gases gas gases (e.g. hydrogen sulphide) Not specific to any one material Less susceptible to poisoning Accuracy and response depend upon High sensitivity to low humidity. concentration of gases Long-term stability
Chemical Carbon dioxide Simple in use Crude identification of specific landfill gas (indicator tubes) Carbon Inexpensive Prone to interference effects. monoxide Should really be used for indication only. Hydrogen sulphide Water vapour Other gases
Photo-ionisation Most organic Very sensitive Susceptible to cross-contamination gases High cost
Source: IWM, 1998
106 Environment Agency Guidance on the management of landfill gas Infra-red detectors Catalytic analysers Infra-red (IR) absorption forms the basis of a range of These have been widely used on landfill sites for the monitoring instruments of varying sophistication. The measurement of low methane concentrations. Gas is most common devices used for landfill gas drawn into the detector and oxidised on a small monitoring are relatively simple devices tuned to heated element (usually a platinum resistance measure IR absorption at specific wavelengths that thermometer) embedded in a pellet (usually are unique to the compound of interest (usually CO2 comprised of ThO2 and Al2O3) covered by a porous and CH4). These conventional analysers comprise an catalytic layer (also known as a pellistor). The IR source, a cell with a defined path length over oxidation reaction on the catalytic surface increases which the measurement is made, a reference cell and the temperature of the pellet, causing the an IR detector. These analysers are generally capable temperature of the platinum element to rise and of providing measurements in the range of 0.5 ppm increases its resistance. Catalytic detectors respond to to 100 per cent CO2 and/or CH4. any flammable gas and oxygen concentrations in the sampled gas are required to exceed 12 per cent by Flame ionisation detectors (FIDs) volume to ensure the complete oxidation of These detectors operate on the principle of flammable components. maintaining a voltage between two electrodes Thermal conductivity detectors located across a small hydrogen flame burning in air. Organic compounds present in the gas drawn into These detectors operate by comparing the thermal the instrument pass through the flame, producing an conductivity of the sampled gas to an electronic increase in the number of free electrons. This registers standard (based on the thermal conductivity of as an increase in the current flowing between the normal atmospheric air). Gas is drawn into the electrodes. FIDs are unsuitable for monitoring in an detector over a catalytically inactive element heated oxygen-deficient environment as their flame requires to a constant temperature (in air). The element is oxygen to support combustion. Although intrinsically contained within a wheatstone bridge circuit, and any safe FIDs have been introduced, the flame poses a changes within the element are registered as a potential ignition source where flammable and voltage change. This is then compared to the unit’s explosive atmospheres are encountered. internal standard. This detector will respond to any gas that has a different thermal conductivity to air. Electrochemical analysers Where the instruments are primarily used to measure These analysers usually comprise an electrochemical methane concentrations, the presence of carbon cell made up of two electrodes immersed in a dioxide may produce falsely low results. It is thus common electrolyte held within an insulated important to calibrate the instrument using CH4 and container. The cell is isolated by a gas permeable CO2 mixtures. membrane or capillary diffusion barrier that allows Semiconductor the component of interest to pass into the cell while retaining the electrolyte. When the component comes Semiconductor sensors operate on the principle of into contact with the electrolyte, the electrical the interaction of gases on the surface of the characteristics of the cell are altered resulting in a semiconductor, which causes a change in the voltage/current. The magnitude of this electrical electrical conductivity. The change in conductivity can change in the cell is proportional to the concentration be displayed as a concentration reading or trigger an of the component, allowing this to be measured. alarm. This type of sensor lacks selectivity for Electrochemical sensors require careful calibration combustible gases and is more often used to detect prior to use to ensure reliability. toxic gases such as hydrogen sulphide. Paramagnetic analysers Chemical Paramagnetic analysers offer an alternative to Chemical sensors were in common use before the electrochemical sensors for oxygen measurement. advent of electronic meters and are usually in the They utilise the positive magnetic susceptibility of form of detector tubes. The gas sample is drawn over oxygen to determine changes in the partial pressure a specifically formulated chemical and, if the of the gas (equated to a concentration). All other compound of interest is present, produces a colour gases, with the exception of nitric oxide, exhibit a change. These tubes can be used only once, and are negative magnetic affinity. subject to interference by other gases and chemical vapours. They can be used for a wide range of gases and vapours. Tubes can vary for the same species, depending on the detection range.
Environment Agency Guidance on the management of landfill gas 107 Photo-ionisation detectors (PIDs) Photo-ionisation detectors use a sealed light source to emit photons of sufficient energy to ionise most organic components of landfill gas. The detector consists of a pair of electrodes within a detector chamber. The sample in the chamber is bombarded by photons, which ionise organic components in the sample. The ions are collected on one of the electrodes. The current this induces in the electrode is measured and processed to give a meter reading. Detection limits for methane are approximately 0.1 per cent, but the detectors can suffer from interference from other organic components being ionised. Other monitoring Gas pressure can be measured using either hand-held or fixed manometers utilising pressure transducers or dial pressure gauges, or using analysers that incorporate pressure monitors. The flow of gas is measured by instruments reading differential pressure, vortex shedding devices, positive displacement meters, mass flow transducers, and van and hot wire anemometers. Greater analytical scope can also be introduced using more sophisticated and expensive instruments including: ● Fourier transform infra-red spectroscopy (FTIR) – these are fast, highly sensitive instruments, capable of measuring absorption phenomena at all wavelengths simultaneously. They are capable of producing detection limits below 1ppm for a wide range of compounds within minutes. ● Long-path monitoring using tuneable lasers are eminently suitable for various forms of gas detection as the tuneable infra-red, ultra-violet and visible lasers posses a number of properties as radiation emitters, including the generation of radiation over a wide spectral range and the emission of high intensity radiation with narrow band width. ● Other long-path monitoring systems use low-energy, non-laser radiation sources, but suffer from poorer sensitivity, provide more limited scope of detection and are operated over much reduced path lengths (up to 500 metres).
108 Environment Agency Guidance on the management of landfill gas Glossary
Acetogenic phase Carburation The period during the decomposition of refuse in a The mixing of air with a volatile fuel to form a landfill when the conversion of organic polymers such as combustible mixture for use in an internal combustion cellulose to simple compounds, such as ethanoic (acetic) engine. and other short chain fatty acids dominates and little or Catalytic oxidation no methanogenic activity takes place. The chemical reaction of oxidation accelerated by a Admixture catalyst. Anything added to form a mixture, or the mixture itself. Cell Adsorption The compartment within a landfill in which waste is The uptake of one substance on to the surface of deposited: The cell has physical boundaries, which may another. be a low permeability base, a bund wall and a low permeability cover. Advection Molecular movement from a region of high pressure to Coal gas one of lower pressure due to the difference in pressure. Gas produced in the old towns gas works by subjecting coal to heat and pressure: The resultant gas was Aerobic collected, purified and distributed through a pipe In the presence of air. network. Anaerobic COD (chemical oxygen demand) In the absence of air. A measure of the total amount of chemically oxidisable Best Available Techniques (BAT) material present in liquid (mass/litre3) . The most effective and advanced stage in the Cohesive soils development of activities and their methods of operation Soils that are primarily composed of clays and can be which indicate the practical suitability of particular moulded. techniques for providing, in principle, the basis for emission limit values designed to prevent and, where Compliance that is not practicable, generally to reduce emissions and The process of achieving conformity with a regulatory the impact on the environment as a whole. standard.
Bentonite Condensate A group of clay minerals that swell on wetting. Forms when warm landfill gas cools during transport or processing (such as compression). Biogas Gas formed by the digestion of organic materials. Consolidated Compacted soft material that has been converted to a Biogenic hard material. For example, sandstones are consolidated Resulting from the actions of living organisms. sands or soils which have been subject to prolonged BOD (biochemical oxygen demand) overburden pressure by overlying strata or mechanical A measure of the amount of material present in water load application. which can be readily oxidised by micro-organisms and is Cover thus a measure of the power of that material to take up Material used to cover solid wastes deposited in landfills. the oxygen in aqueous media (mass/litre3). Daily cover may be used at the end of each working day Borehole to minimise odours, wind-blown litter, insect or rodent A hole drilled outside wastes for the purposes of infestation, and water ingress. Final cover is the layer or monitoring or sampling. layers of materials placed on the surface of the landfill before its restoration. Capping material A landfill covering, usually having a low permeability to water. Permanent capping is part of the final restoration following completion of landfill/tipping. Temporary capping is an intermediate cap, which may be removed on the resumption of tipping.
Environment Agency Guidance on the management of landfill gas 109 Construction Quality Assurance (CQA) Gas chromatography A planned and systematic application and recording of Analytical technique for separating gas mixtures, in methods and actions designed to provide adequate which the gas is passed through a long column confidence that items or services meet contractual and containing a fixed absorbent phase that separates the regulatory requirements, and will perform satisfactorily in gas mixture into its component parts. service. Gas drainage layer Decomposition An engineered layer of high gas permeability Natural breakdown of materials by the action of micro- immediately underlying an artificially established cap organisms, chemical reaction or physical processes. that is designed to facilitate the collection of landfill gas.
Degradation Geomembrane/synthetic See decomposition. An engineered polymeric material fabricated to a low hydraulic permeability. Differential pressure A difference between the pressures at two distinct points Geotextile within a system. A geosynthetic material normally from man-made fibres which is fabricated to be permeable. Diffusive flow Molecular movement from a region of high Groundwater concentration to a more dilute region of low All water that is below the surface of the ground and in concentration due to random, free movement of the direct contact with the ground or subsoil. molecules. Hydrocarbon Emission A chemical compound containing only hydrogen and The direct or indirect release of substances, vibrations, carbon atoms. heat or noise from individual or diffuse sources in an Hydrolysis installation into the air, water or land. A chemical reaction in which water reacts with another Endogenous substance and gives decomposition or other products. Developing or originating within an organism, or part of Inert an organism. Strictly, material or wastes that will not undergo any Environmental impact significant physical, chemical or biological The total effect of any operation on the environment. transformations. Inert waste will not dissolve, burn or otherwise physically or chemically react, biodegrade or Faults adversely affect other matter which comes into contact Fractures in rock along which relative displacement has in any way likely to give rise to environmental pollution occurred. or harm to human health. The total leachablity and Flame ionisation detector (FID) pollutant content of waste and the ecotoxicity of the Detector based on the conduction by ions produced leachate must be insignificant and, in particular, not when the analyte is ionised in a hydrogen/air flame. The endanger the quality of surface and/or groundwater resulting voltage change is proportional to the Infra-red detector concentration of the analyte. An instrument that measures adsorption in the infra-red Flame ionisation range of spectrum. Method of detection that measures flammable gas by Installation (as defined by the PPC (England and Wales) ionising a sample in a hydrogen flame. Regulations 2000) (PPC Scotland Regulations 2003) Flammable substance A stationary technical unit where one or more activities A substance supporting combustion in air. listed in Part 1 of Schedule 1 are carried out.
Flow velocity Any other location on the same site where any other The flow rate (litre3/time) divided by the cross-sectional directly associated activities are carried out which have a area of the pipe (in litre2). technical connection with the activities carried out in the stationary technical unit and which could have and Flux box effect on pollution. A chamber that, when sealed against a landfill surface, allows surface emissions to enter by diffusion as a result And, other than in Schedule 3, references to an of the concentration gradient between the landfill installation include references to part of an installation. surface and atmosphere.
110 Environment Agency Guidance on the management of landfill gas Interstitial Monitoring Occurring in the interstices (spaces) between other A continuous or regular periodic check to determine the material. ongoing nature of the potential hazard, conditions along environmental pathways and the environmental impacts Intrinsically safe of landfill operations to ensure the landfill is performing Said of apparatus that is designed to be safe under according to design. The general definition of dangerous conditions – usually refers to equipment that monitoring includes measurements undertaken for can be used in an explosive atmosphere because it will compliance purposes and those undertaken to assess not produce a spark. landfill performance. Joints Odorants Cracks within a block of rock along which there has Strictly, chemical compounds added to mains gas to been little or no movement of the rock. impart odour or, more widely, particularly odorous Landfill gas volatile organic compounds in landfill gas. All the gases generated from the landfilled waste. Odour threshold value (odour detection threshold) Leachate The concentration of an odorous gas, detected by 50 Any liquid percolating through the deposited waste and per cent of an odour panel. emitted from or contained within a landfill. Partial pressure Leachate recirculation In a mixture of gases or vapours, each constituent can The practice of returning leachate to the landfill from be considered to contribute to the total pressure that which it has been abstracted. pressure it would exert if it were present alone in a vessel of the same volume as that occupied by the mixture LEL (Lower Explosive Limit) (units: %). The lowest percentage concentration by volume of a flammable substance in air which will allow an explosion Permeability to occur in a confined space at 25oC and normal A measure of the rate at which a gas will pass through a atmospheric pressure, and where an ignition source is medium (litre2). The coefficient of permeability of a present (units: %). given fluid is an expression of the rate of flow through unit area and thickness under unit differential pressure at Liner a given temperature (litre/time). A natural or synthetic membrane material, used to line the base and sides of a landfill site to reduce the rate of pH leachate and gas emissions. An expression of hydrogen ion concentration, specifically, the negative logarithm of the hydrogen ion Lithology concentration. The range is from 0 to 14, with 7 as The study and description of the general, gross physical neutral, 0–7 as acidic, and 7–14 as alkaline. characteristics of a rock. Phase (of a landfill) Mains gas A prepared operational, temporarily restored or restored A commercial methane-rich gas distributed through area. underground pipes to domestic, commercial and industrial customers. Pollution, pollutant The addition of materials or energy to an existing Marsh gas environment system to the extent that undesirable Gas produced from marshes and bogs. changes are produced directly or indirectly in that Methane system: a pollutant is a material or type of energy whose The hydrocarbon of the highest concentration typically introduction into an environmental system leads to found in landfill gas (CH4). pollution. Methanogenesis Polycyclic The process leading to the production of methane. Organic chemical compound where the atoms form more than one ring structure. Moisture content Percentage of water contained in a sample of waste or Potentiation soil, usually determined by drying the sample at 105oC The capacity of a compound to act as precursor for to constant weight. sensory or toxic effects from other compounds without causing such effects itself.
Environment Agency Guidance on the management of landfill gas 111 ppb Stoichiometric Parts per billion, method of expressing concentration. The exact proportions in which substances react. For 1 ppb is a thousandth of a ppm (see below). combustion, a theoretical minimum amount of air or oxygen required to consume the fuel completely. ppm Parts per million, method of expressing concentration. Synergism 10,000 ppm v/v equates to 1 per cent gas at standard An interaction of elements such that their combined temperature and pressure (STP) by volume. effect is greater than the sum of their individual effects.
ppmv Time-weighted average Part per million by volume. The resultant value averaged over a period of time.
Protocol Transitory A formal or customary procedure. Not permanent, short lived.
Putrescible Trigger/action levels A substance capable of being readily decomposed by Trigger levels are compliance levels and, in order to meet bacterial action. Offensive odours usually occur as by- trigger levels, action levels should be set at a level at products of this decomposition. which the operator can take action to remain compliant. These may form part of the PPC permit. Retention time The time at which the gases stay within the shroud at, or UEL (Upper Explosive Limit) above, a specific temperature (also known as residence The highest concentration of mixture of a compound time) (measured in units of time). and air which will support an explosion at 25oC and normal atmospheric pressure, and in the presence of a Settlement flame. The amount by which a landfill surface sinks below its original level due to ravelling, compaction by its own v/v weight, and degradation of the waste, e.g. a tipped By volume (as in % v/v or ppm v/v); usually applied to waste thickness of 40 metres settling by 8 metres would gases. have undergone 20 per cent settlement. Well head Sewer gas The top portion of a well, usually containing a valve, and Gas produced by the decomposition of organic various monitoring parts. compounds in sewerage. Well Soft development A hole drilled within wastes for the purposes of The re-use of land that avoids domestic, industrial or sampling, monitoring, gas or leachate extraction. commercial property. Wobbe index Specific moisture content Ratio of the corresponding calorific value of a gas per Specific moisture content of air is the ratio of the mass of unit volume and the square root of its relative density water to the mass of dry air in a given volume of moist under the same reference conditions. It is dimensionless. air (units: %). Working area Spike survey The area or areas of a landfill in which waste is currently Measurement of methane gradient with one data point being deposited. at the base of the spiked hole and an assumed zero w/w concentration at the surface. By weight (as in % w/w). Stabilisation Zone As applied to landfill, this term includes the degradation Part of the site surface deemed to be of generally of organic matter or the leaching of inorganic matter to uniform character such that the area concerned is stable products and the settlement of the fill to its rest assumed to be suitably homogenous in the context of level. It also refers to the use of plants and/or geotextiles surface emissions. to prevent soil erosion from the surface of a landfill or spoil heap.
Stratification Formation of distinct layers due to ineffective mixing of gases.
112 Environment Agency Guidance on the management of landfill gas Acronyms
ATP Adenosine triphosphate IPPC Integrated Pollution Prevention and Control BAT Best Available Technique IWM Institute of Wastes Management (now Chartered Institute of Wastes Management) BES Bentonite-enhanced soil LEL Lower Explosive Limit BMW Biodegradable municipal waste LLDPE Linear low density polyethylene BOD Biochemical oxygen demand LPA Local planning authority CFC Chlorofluorocarbon LPG Liquefied petroleum gas COD Chemical oxygen demand MDPE Medium density polyethylene COSHH Control of Substances Hazardous to Health NB Nominal bore CQA Construction Quality Assurance Netcen National Environmental Technology Centre DAC Dense asphaltic concrete NFFO Non-fossil Fuel Obligation Defra Department for Environment, Food and Rural Affairs NMVOC Non-methane volatile organic compound
DETR Department of the Environment, Transport NOx Nitrogen oxides and the Regions ODPM Office of the Deputy Prime Minister DoE Department of the Environment OEL Occupational Exposure Limit Dti Department of Trade and Industry OES Occupational Exposure Standard EAL Environmental Assessment Level PAH Polycyclic aromatic hydrocarbon EIA Environmental Impact Assessment PC Predicted concentration EPAQS Expert Panel on Air Quality Standards PCB Polychlorinated biphenyl EQS Environmental Quality Standard PCDD Polychlorinated dibenzo-p-dioxin ETSU Energy Technology Support Unit (now part PCDF Polychlorinated dibenzofuran of Future Energy Solutions, AEA Technology) PEC Predicted Environmental Concentration FID Flame ionisation detector PID Photo-ionisation detector GCL Geosynthetic clay liner PP Polypropylene GC–MS Gas chromatography–mass spectrometry PPC Pollution Prevention and Control GDPO General Development Procedure Order PPG Planning Policy Guidance GLC Ground level concentration SEPA Scottish Environment Protection Agency GRT Gas retention time SO Sulphur oxides GWP Global warming potential x SSSI Site of Special Scientific Interest HCl Hydrogen chloride TOC Total organic carbon HCFC Hydrochlorofluorocarbon TWA Time-weighted average HDPE High density polyethylene UEL Upper Explosive Limit HF Hydrogen fluoride VOC Volatile organic compound HRT Hydraulic retention time WHO World Health Organization HSE Health and Safety Executive
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Environment Agency Guidance on the management of landfill gas 115 Environment Agency (2003c) Interpretation of the Institute of Waste Management (IWM) (1998) The engineering requirements of Schedule 2 of the Landfill monitoring of landfill gas. 2nd edition. Landfill Gas (England and Wales) Regulations 2002. Landfill Monitoring Working Group report. IWM, Regulatory Guidance Note 6 Version 4. Environment Northampton. Agency, Bristol. Johnston, A.G., Edwards, J.S., Owen, J., Marshall, Environment Agency (2004a) Guidance on landfill S.J. and Young, S.D. (2000) Research into methane completion. Environment Agency, Bristol. emissions from landfills. In: Proceedings of Waste 2000: waste management at the dawn of the Third Environment Agency (2004b) Guidance for Millennium, held Stratford-upon-Avon. pp. 13–20. monitoring enclosed landfill gas flares. Environment Agency, Bristol. Knox, K. (1990) The relationship between leachate and gas. In: Proceedings of international landfill gas Environment Agency (2004c) Guidance on gas conference, Energy and Environment ’90. Energy treatment technologies for landfill gas engines. Technology Support Unit/Department of the Environment Agency, Bristol. Environment. pp. 367–386. Environment Agency (2004d) Guidance for Latham, B. (1998) Understanding landfill gas monitoring landfill gas surface emissions. Environment monitoring and control: identification of landfill gas and Agency, Bristol. other biogas sources. Environmental Services Environment Agency (2004e) Guidance for Association. Course Notes 11–12 November 1998. monitoring landfill gas engine emissions. Environment ESA, London. Agency, Bristol. Meadows, M.P. and Parkin, M. (1999) Determination Environment Agency (2004f) Guidance for of methane emissions from north west landfills. Report monitoring trace components in landfill gas. No. T558/1. Environment Agency, Warrington. Environment Agency, Bristol. National Environmental Technology Centre Environment Agency (2004g) Monitoring of (Netcen) (1998) National atmospheric emissions particulate matter in ambient air around waste inventory for 1998. Available from the UK National Air management facilities. Technical Guidance Document Quality Information Archive. (Monitoring) M17. Environment Agency, Bristol. http://www.airquality.co.uk/archive/index.php. Farquhar, G.J. and Rovers, F.A. (1973) Gas National Environmental Technology Centre production during refuse decomposition. Water, Air (Netcen) (2000) The UK National Air Quality and Soil Pollution 2, pp. 483-493. Information Archive. http://www.airquality.co.uk/archive/index.php. Fisher, A. J. (1992) The lubrication of landfill gas engines. Proceedings of Power Generation from Office of the Deputy Prime Minister (1999) Landfill Gas Workshop. Planning Policy Guidance Note 10: Planning and waste management. The Stationery Office, London. Forbes, C.D., McGuigan, M.J. and Kerr, T. (1996) The US LFG industry: current status and future trends. Office of the Deputy Prime Minister (ODPM) In: Proceedings of international landfill conference on (2002) Planning Policy Guidance Note 23: Planning and best practice and future opportunities. Published by pollution control. Consultation paper, July 2002. ETSU for the Department of Trade and Industry. ODPM, London. Gendebien, A., Pauwels, M., Constant, M., Ledrut- O’Neil, M.J., Smith, A., Heckelman, P.E. and Damanet, M.-J. and Willumsen, H.C. (1992) Landfill Obenchain J.R. Jr. (eds) (2001) The Merck Index: an gas: from environment to energy. Final report to the encyclopaedia of chemicals, drugs and biologicals. 13th Commission of the European Communities. ISBN 9 edition. Merck & Co. Inc., Whitehouse Station, New 28 263672 0. European Commission, Luxembourg Jersey. Hartless, R.P. (2000) Developing a risk assessment Pacey, J. and Augenstein, D. (1990) Modelling landfill framework for landfill gas - incorporating meteorological methane generation. In: Proceedings of international effects. In: Proceedings of Waste 2000: waste landfill gas conference, Energy and Environment ’90. management at the dawn of the Third Millennium, Energy Technology Support Unit/Department of the held Stratford-upon-Avon. pp. 41–50. Environment. pp. 223–267. Health and Safety Executive (HSE) (2002) Guidance Note EH40: occupational exposure limits 2002. HSE Books, Sudbury.
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Environment Agency Guidance on the management of landfill gas 117 Index
abatement order 13 and temperature 62 accidents and failures boreholes, monitoring 88-90 emergency procedure examples 103, 104 and Gas Management Plan 37-8 calcium sulphate in landfills 58 liners 71 capping, materials and recommendations 41, 71 risk assessment 31-4 carbon dioxide 51 acetogenesis 56-7 and asphyxiation 53-4 acid gases 54, 55 in combustion emissions 67 see also corrosion and ecotoxicity 29 acid souring 62 effects on methane flammability 101 acronyms 113 emergency procedure 104 advective flow 65, 66 global warming potential 34-5 aerobic microbial activity 56 physiological effects 54 air quality solubility 53 environmental benchmarks 23 sub-surface migration 23, 29, 59 monitoring 90 trigger levels 89-90 standards and impact assessment 12, 31 carbon monoxide 23, 67 and World Health Organization guidelines 29 carbonic acid 54 Air Quality Strategy for England and Wales 12, 31 catalytic analysers 105, 107 anaerobic microbial activity 56-7, 58 CFCs 55, 59 asphyxiation 24, 53-4 chemical sensor 106, 107 atmospheric pressure Clean Air Act 13 and boreholes 89 closed landfill sites 8, 9 and gas migration 66-7 see also completion monitoring 85-6, 92, 108 collection of gas best practice 21 barometric pressure see atmospheric pressure estimating efficiency 21 benchmarks risk assessment 31-4 environmental 23 see also pipework and pipelines; wells indicative for landfill gas utilisation 22 combustion of gas 67 Best Available Technique (BAT) 8-9 see also explosions and fires gas engines and flares 31 completion 29 biodegradable waste 7 and Gas Management Plan 38-9 best practice requirements 21-2 composition of gas calculating gas production rates 20-2 landfill 51-3, 58-9, 100 density effects 60 and risk assessment 28-9 gas generating components 59-60 various sources compared 100 government targets 59 compressors and boosters 77-8 moisture content and distribution 60 computer models 25, 29-30 biodegradation conceptual site model effect of pH 61-2 environmental benchmarks 23 stages in 56-8 gas emission and migration 24-7
118 Environment Agency Report title hazard identification and risk screening 17, 20 electrochemical ionisers 105, 107 monitoring and review inputs 33 emergency procedures overview 19 examples 103, 104 pathways 23-4 and Gas Management Plan 38 receptors 22-3 see also accidents and failures source of risk 20-2 emissions of gas condensate calculating approximate flow 20, 62 acidity 54 effect of atmospheric pressure 66-7 management 78-9 from flaring and utilisation 67-8, 90 and pipelines 75 insignificant defined 26 secondary source of methane 53 lateral 65-7, 88-9 construction quality assurance (CQA) plan 69-70 modelling 25, 29-30, 65 containment monitoring 86-92 best practice 21 pathways 23-4, 63-4, 66 principles and regulatory requirements 40-1 point sources 23-4, 67, 71 risk assessment 31-3 potential impacts 68 see also capping; lining standards 10 contaminated land defined 8 surface 64-5, 86-8 control measures Emissions Review 9-10 capping 41, 71 engines, gas 25, 26, 31, 81 collection wells 72-7 see also utilisation condensate management 78-9 Environmental Assessment Levels construction quality assurance (CQA) plan 69-70 air quality 23, 90 design of systems 69, 72 and impact assessment 30-1 extraction plant 77-8 environmental benchmarks 23-4, 29 flaring 81-3 Environmental Protection Act 13 and Gas Management Plan 37, 84 Environmental Quality Standard 30-1 lining 70-1 Environmental Statement 10 operating and maintenance procedures 84 explosions and fires phasing of site development 70 air ingress concerns 77 process control systems 80 effect on landfill gas composition 62 treatment and supplementary processing 80, 82, 83 equation for combustion of methane 66 utilisation 80-1, 82, 83 flammability and explosivity limits 53, 101 control valves 80 Loscoe explosion 66 corrosion 54-5 risk assessment 24, 31-2 and condensate 79 extraction plant 77-8 and utilisation plants 81 COSHH 13 failures see accidents Fick’s Law 66 Darcy’s Law 66 field measurements 92-4 data analysis and reporting 92 fires see under explosions decomposition of biodegradable waste 56-8 flame ionisation detectors (FIDs) 86, 88, 105, 107 density of landfill gas 53 flammability see under explosions diffusive flow 66 flares and flaring dispersion and emission rates 24-7 best practice 21 ecotoxicity 29, 55
Environment Agency Report title 119 composition and impact of emissions 67-8, 102 hazardous waste 7 design elements 82 hazards emission standards 31 identification and risk assessment 19 gas treatment and processing 42, 80 potential of landfill gas 24 monitoring 90 health and safety open diffusion flares 81 and condensate 79 regulatory requirements 43 COSHH 13 stack height 13, 25, 26, 82 and odour 12 support fuels 82-3 hydraulic retention time 61 flux box 86, 87, 88 hydrogen chloride 55, 68 Fourier transform infra-red spectroscopy 108 hydrogen fluoride 68 hydrogen sulphide gas control formation in landfills 58 key measures and regulatory requirements 7-8, 40-4 and gas engines 81 timing of extraction 21 as marker 92 see also collection; containment; flares; odour 55 utilisation toxicity 54 gas engines 25, 26, 31, 81 hydrolysis of biodegradable waste 56-7 see also utilisation gas flow calculations 20-1 impact assessment 30-1 Gas Management Plan inert waste landfills 20 accidents and consequences 33 infra-red detectors 105, 107 best practice requirements 21-2 inorganic waste 51 completion and aftercare 38-40 and gas emission 22 control measures 37 sulphate-based 58 corrosion 54-5 instruments and sampling 92-4, 105-8 definition and key elements 36 Integrated Pollution Prevention and Control (IPPC) emergency procedures and protocols 38 Directive 7, 8-9 failure and accident action plan 37-8 gas collection 72 Landfill Directive 7 and monitoring procedures 37, 92 and biodegradable waste targets 59 plant operation and maintenance 84 capping recommendations 71 point source emissions 67 lining requirements 70 and risk assessment 17 monitoring requirements 44-5 waste inputs 36-7 Landfill (England and Wales) Regulations 7, 8, 44 gas sensors 94, 105, 105-8 landfill gas defined 7, 56 GasSim model 25, 30 Landfill Regulations and gas control 7-8, 70, 88 generation of gas see production Landfill (Scotland) Regulations 7, 8 geomembranes 71 landfill sites global warming potential (GWP) 29, 34-5, 55 classes of 7, 20 glossary 109-12 closed 8, 9 ground level concentration (GLC) estimates 25-6 completion 29 ground-based gases 52 conceptual model 19-27 gypsum in landfills 58 development on or adjacent to 10-11 gas generating components in waste 59-60 inorganic waste 22, 51, 58
120 Environment Agency Report title licensed and unlicensed 8 see also atmospheric pressure lining and capping 41-2, 70-1 methane 7, 51 planning and development 10, 70 combustion 67 PPC permits 9 and ecotoxicity 29 unlined 71 emergency procedure 103 lateral emissions, monitoring 88-90 flammability and explosivity 53, 101 leachate global warming potential 34, 55 and excess moisture 61 monitoring lateral emission 88-90 monitoring 85 monitoring surface emission 86-8 and pipelines 75 oxidation 29, 82 recirculation 61, 63 and photochemical pollution 55-6 secondary source of methane 53 production process by microbial action 56 leaks see emissions solubility 53 legislation sub-surface migration 23, 59 Air Quality Strategy for England and Wales 12 trigger levels 89-90 Clean Air Act 13 UK emission totals 62 COSHH 13 methanogenesis 57, 59 and Environment Agency strategy 9 inhibition by chemical agents 62 Integrated Pollution Prevention and Control optimum pH 61-2 (IPPC) Directive 7 microbial oxidation 82 Landfill Directive 7 microbial seeding 63 Landfill (England and Wales) Regulations 7 mine gas 100 Landfill (Scotland) Regulations 7 modelling, computer 25, 29-30, 65 overview of current legislation 11-12 moisture content and optimum gas production 63 planning and development 11 monitoring Pollution Prevention and Control (PPC) air quality 90 regulations 7 boreholes 88-90 Waste Framework Directive 7 combustion 90 Waste Management Licensing Regulations 7 data analysis and reporting 92, 94 licensing of landfill sites 8 frequency 45-6, 89 lining 21 instruments and sampling techniques 92-4 Landfill Regulations 40-1, 70 lateral emissions 88-90 materials 70-1 legislative guidance 44-7 local authorities main objectives 33-4, 44 and air quality control 12-13 meteorological monitoring 92 planning and development 10-11 pressure 85-6 and unlicensed sites 8 procedures 92 long-path monitoring 108 regulatory requirements 44-7 Loscoe gas explosion 66 at site preparation phase 46 source monitoring 85 maintenance and operational procedures 84 surface emissions 86-8 marsh gas 100 trace elements 86, 94 meteorological conditions trigger levels for methane and carbon dioxide 89 and gas dispersion 30 type and frequency of sampling 47 monitoring 92 and odour 55
Environment Agency Report title 121 National Planning Policy Guideline 10 calculations 26-7 natural gas 100 and impact assessment 30 nitrogen oxides 23, 55-6, 67-8 pressure non-biodegradable waste 20, 22, 51 and boreholes 89 non-hazardous waste 7 control 80 non-methane volatile organic compounds (NMVOCs) manometers 108 67, 68 testing of pipework 76 total 86 odour and wells 85-6 defined 90 see also atmospheric pressure monitoring and action plan 90-2 production of gas and rate of gas production 62-3 calculating approximate gas flow 20, 62 statutory nuisance 12-13 estimating rates and modelling 62, 63 substances responsible 55, 58 factors affecting production 59-63 operational and maintenance procedures 84 microbial action 56-8 oxides of sulphur 23, 68 optimising conditions 62-3 ozone 31, 35, 56 timing of extraction 21 see also emission paramagnetic analysers 105, 107 particulates 23, 68 receptors monitoring 92-3 identified 22-3 pathway, emission 23-4, 63-4, 66 pathways 23-4 pH prioritisation and impact assessment 24 and biodegradation 61-2 and toxicological assessment 29 and corrosion 54-5 regulations see legislation photo-ionisation detectors (PIDs) 106, 108 risk assessment 17-36 photochemical pollution 55-6 accidents and failures 31-4 pin wells 72, 73 bulk and composition assessment 28-9 pipework and pipelines 75-7, 79 complex risk 28 point sources 23-4, 71 computer models 25, 29-30 pollutants and development of Gas Management Plan 18 and air quality control 12, 23, 31 emission and dispersion rate calculations sources of emission 63-4 24-7 and utilisation plants 81 environmental benchmarks 23, 29 see also trace components global warming potentials 34-5 pollution, photochemical 55-6 hazard identification 19 Pollution Prevention and Control (England and impact assessment 30-1 Wales) Regulations 8-9 and Landfill Regulations 18 Pollution Prevention and Control (Scotland) monitoring and reviews 33 Regulations 7, 8 previously landfilled areas 22 polyaromatic hydrocarbons (PAHs) 31 receptors 22-3, 24 polychlorinated dibenzofurans (PCDFs) 68 simple risk 28 polychlorinated dibezodioxins (PCDDs) 68 source of risk 20-2 PPC permits 9, 10, 17, 22, 40 strategy and general framework 17-18 Predicted Environmental Concentration (PEC) Tier 2 and 3 risk levels 10, 17, 27-8 and accidents 32
122 Environment Agency Report title sampling techniques and instruments 92-4, 105-8 treatment and supplementary processing 80, 81 semiconductor sensor 106, 107 waste sensors, gas 94, 105-8 composition 59-60 settlement and gas wells 74 existing deposits 22 site model see conceptual site model see also biodegradable; inert; inorganic solubility of landfill gas 53 Waste Framework Directive 7 sources of gas Waste Management Licensing Regulations 7, 8 compositional comparison 100 water and moisture monitoring 85 effects on gas production 60-1 point sources 23-4, 71 see also condensate; leachate see also emissions wells, collection stack height 13, 25, 26, 82 collection layers 75 statistical analysis 94 compressors and boosters 77-8 sub-surface migration design of wells 72, 74 and ecotoxicity 29, 55 head arrangements 74 effects on gas composition 59 layout and spacing 73 factors affecting flow 65-7, 71 monitoring techniques 85-6 risk assessments 21, 23, 24, 27 patterns for well fields 76 sulphate in landfills 58 performance reduction and failure 75 sulphur dioxide 23, 68 pin wells 72, 73 support fuels 82-3 pipework 75-7 surface emissions 64-5 retrofitting 74 monitoring 86-8 wells, monitoring see boreholes temperature and gas production 62 thermal conductivity detectors 106, 107 Tier 2 and 3 risk levels 10, 17, 27 topography 30 Town and Country Environmental Impact Assessment Regulations 10 Town and Country Planning Act 10 toxicity risk and assessment 24, 29, 54 trace components 96-8 monitoring 86 treatment of gas 21, 42, 43, 80 trigger levels 89-90, 94 unlicensed landfill sites 8 unlined landfills 71 utilisation of gas best practice 21 criteria for viability 42 emissions from combustion systems 67-8 indicative benchmarks 21-2 key factors in plant design 80-1 options for use 81, 83
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